text: #pragma comment(linker, "/stack:200000000")
  #pragma GCC optimize("Ofast,no-stack-protector")
  #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
  #pragma GCC optimize("unroll-loops")
  
  #define _CRT_SECURE_NO_WARNINGS
  
  # include <bits/stdc++.h>
  # include <iostream>
  # include <cmath>
  # include <algorithm>
  # include <stdio.h>
  # include <cstring>
  # include <string>
  # include <cstdlib>
  # include <vector>
  # include <bitset>
  # include <map>
  # include <queue>
  # include <ctime>
  # include <stack>
  # include <set>
  # include <list>
  //#include <ext/pb_ds/detail/standard_policies.hpp>
  //#include<ext/pb_ds/assoc_container.hpp>
  //#include<ext/pb_ds/tree_policy.hpp>
  //using namespace __gnu_pbds;
  using namespace std;
  
  #define ui              unsigned int
  #define ll              long long int
  #define ull             unsigned long long int
  #define ld              long double
  #define pii             pair<int,int>
  #define pll             pair<ll,ll>
  #define vi              vector<int>
  #define vll             vector<ll>
  #define vc              vector<char>
  #define vs              vector<string>
  #define vpii            vector<pii>
  #define vpll            vector<pll>
  #define sti             stack<int>
  #define stll            stack<ll>
  #define qi              queue<int>
  #define qll             queue<ll>
  #define sei             set<int>
  #define sell            set<ll>
  #define sec             set<char>
  #define ses             set<string>
  #define pqi             priority_queue<int>
  #define pqgrt           priority_queue<int,vector<int>,greater<int> >
  #define mpii            map<int,int>
  #define mpll            map<ll,ll>
  #define mpsi            map<string,int>
  #define mpis            map<int,string>
  #define mpci            map<char,int>
  #define umpii           unordered_map<int,int>
  #define umpll           unordered_map<ll,ll>
  #define umpsi           unordered_map<string,int>
  #define umpis           unordered_map<int,string>
  #define umpci           unordered_map<char,int>
  #define ump             unordered_map
  #define use             unordered_set
  #define pq              priority_queue
  
  #define F               first
  #define S               second
  #define pb              push_back
  #define pp              pop_back
  #define UB              upper_bound
  #define LB              lower_bound
  
  #define pf                  printf
  #define sfc(a)              scanf("%c",&a)
  #define sfs(a)              scanf("%s",&a)
  #define sfi1(a)             scanf("%d",&a)
  #define sfi2(a,b)           scanf("%d %d",&a,&b)
  #define sfi3(a,b,c)         scanf("%d %d %d",&a,&b,&c)
  #define sfi4(a,b,c,d)       scanf("%d %d %d %d",&a,&b,&c,&d)
  #define sfd1(a)             scanf("%lf",&a)
  #define sfd2(a,b)           scanf("%lf %lf",&a,&b)
  #define sfd3(a,b,c)         scanf("%lf %lf %lf",&a,&b,&c)
  #define sfd4(a,b,c,d)       scanf("%lf %lf %lf %lf",&a,&b,&c,&d)
  #define sfll1(a)            scanf("%lld",&a)
  #define sfll2(a,b)          scanf("%lld %lld",&a,&b)
  #define sfll3(a,b,c)        scanf("%lld %lld %lld",&a,&b,&c)
  #define sfll4(a,b,c,d)      scanf("%lld %lld %lld %lld",&a,&b,&c,&d)
  #define sfull1(a)           scanf("%llu",&a)
  #define sfull2(a,b)         scanf("%llu %llu",&a,&b)
  #define sfull3(a,b,c)       scanf("%llu %llu %llu",&a,&b,&c)
  #define sfull4(a,b,c,d)     scanf("%llu %llu %llu %llu",&a,&b,&c,&d)
  
  #define ang(a, b, c)        acos((b*b+c*c-a*a)/(2.0*b*c))
  #define dist(x1,y1,x2,y2)   sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2))
  #define rev(v)              reverse(v.begin(),v.end())
  #define srt(v)              sort(v.begin(),v.end())
  #define grtsrt(v)           sort(v.begin(),v.end(),greater<pll>())
  #define all(v)              v.begin(),v.end()
  #define minv(v)             *min_element(v.begin(),v.end())
  #define maxv(v)             *max_element(v.begin(),v.end())
  #define cntv(v,a)           count(v.begin(),v.end(),a)
  #define one(x)              __builtin_popcount(x)
  #define valid(tx,ty)        tx>=0&&tx<r&&ty>=0&&ty<c
  #define mem(arr,val)        memset(arr,val,sizeof(arr))
  #define Unique(x)           x.erase(unique(x.begin(),x.end()),x.end())
  #define fast                ios_base::sync_with_stdio(false),cin.tie(NULL),cout.tie(NULL)
  #define forstl(it,v)        for(__typeof(v.begin()) it=v.begin();it!=v.end();++it)
  #define loop(i,n)           for(int i=0;i<n;i++)
  #define loop1(i,n)          for(int i=1;i<=n;i++)
  #define case_print          printf("Case %d: ",cs)
  
  template <class T1> void deb(T1 e1){cout<<e1<<endl;}
  template <class T1,class T2> void deb(T1 e1,T2 e2){cout<<e1<<" "<<e2<<endl;}
  template <class T1,class T2,class T3> void deb(T1 e1,T2 e2,T3 e3){cout<<e1<<" "<<e2<<" "<<e3<<endl;}
  template <class T1,class T2,class T3,class T4> void deb(T1 e1,T2 e2,T3 e3,T4 e4){cout<<e1<<" "<<e2<<" "<<e3<<" "<<e4<<endl;}
  template <class T1,class T2,class T3,class T4,class T5> void deb(T1 e1,T2 e2,T3 e3,T4 e4,T5 e5){cout<<e1<<" "<<e2<<" "<<e3<<" "<<e4<<" "<<e5<<endl;}
  template <class T1,class T2,class T3,class T4,class T5,class T6> void deb(T1 e1,T2 e2,T3 e3,T4 e4,T5 e5,T6 e6){cout<<e1<<" "<<e2<<" "<<e3<<" "<<e4<<" "<<e5<<" "<<e6<<endl;}
  
  template <typename T> T gcd(T a, T b){while (b){T t = a % b;a = b;b = t;}return a;}
  template <typename T> T lcm(T a, T b){return a / gcd(a, b) * b;}
  template <typename T> T gcd(T a, T b, T c){return gcd(gcd(a, b), c);}
  template <typename T> bool isPrime(T x){if (x<=4||x%2==0||x%3==0)return x == 2 || x == 3;for (T i = 5; i * i <= x; i += 6) if (x % i == 0 || x % (i + 2) == 0) return 0;return 1;}
  template <typename T> bool bitcheck(T p,T pos){return (bool)(p&(1<<pos));}
  template <typename T> T biton(T p,T pos){return p=p|(1<<pos);}
  template <typename T> T bitoff(T p,T pos){return p=p&~(1<<pos);}
  template <typename T> ll toint(T s) {ll p;stringstream ss(s); ss>>p; return p;}
  template <typename T> string tostring(T n) {stringstream ss; ss << n; return ss.str();}
  template <typename T> T POW(T b,T p) {T Ans=1; while(p){if(p&1)Ans=(Ans*b);b=(b*b);p>>=1;}return Ans;}
  template <typename T> T BigMod(T b,T p,T Mod) {T Ans=1; while(p){if(p&1)Ans=(Ans*b)%Mod;b=(b*b)%Mod;p>>=1;}return Ans;}
  template <typename T> T ModInverse(T p,T Mod) {return BigMod(p,Mod-2,Mod);}
  
  const int fx[]={+0,+0,+1,-1,-1,+1,-1,+1};
  const int fy[]={-1,+1,+0,+0,+1,+1,-1,-1};
  
  #define mx              100010
  #define inf             0x3f3f3f3f
  #define mod             1000000007
  #define PI              2*acos(0.0)
  #define E               2.71828182845904523536
  const double eps        = 1e-9;
  //template <typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
  //template <typename T> using ordered_set2 = tree<pair <T, string>, null_type, less<pair <T, string>>, rb_tree_tag, tree_order_statistics_node_update>;
  //ll histogram(ll arr[],ll n){ll top,i=0,sum=0,max_area=0;stack<ll>st;st.push(0);while(i<n){if(st.empty()||arr[st.top()]<=arr[i]) st.push(i++);else{top=st.top();st.pop();if(st.empty())sum=arr[top]*i;else sum=arr[top]*(i-st.top()-1);max_area=max(sum,max_area);}}while(!st.empty()){top=st.top();st.pop();if(st.empty()) sum=arr[top]*i;else sum=arr[top]*(i-st.top()-1);max_area=max(sum,max_area);}return max_area;}
  //ll ext_gcd(ll a,ll b,ll *x,ll *y){if(a==0){*x=0;*y=1;return b;}ll x1,y1;ll gcd = ext_gcd(b%a,a,&x1,&y1);*x=y1-(b/a)*x1;*y=x1;return gcd;}
  //bool mark[mx];vector<ll>prime;
  //void sieve(){memset(mark,true,sizeof(mark));for(ll i=3;i<=(ll)sqrt(mx);i+=2){if(mark[i]){for(ll j=i*i;j<=mx;j+=2*i) mark[j]=false;}}prime.push_back(2);for(ll i=3;i<=mx;i+=2) if(mark[i]) prime.push_back(i);}
  ///code start here
  
  
  int main(){
  //    freopen("Input.txt","r",stdin); freopen("Output.txt","w",stdout);
      string s,t;
      cin>>s>>t;
      int cnt1=0;
      loop(i,s.size()){
          if(s[i]=='1'&&t[i]=='0') cnt1++;
          if(t[i]=='1'&&s[i]=='0') cnt1++;
      }
      if(cnt1&1){
          cout<<"impossible\n";
      }
      else{
          string ans="";
          int cnt2=0;
          loop(i,s.size()){
              if(s[i]==t[i]) ans+=s[i];
              else if(s[i]!=t[i]){
                  cnt2++;
                  if(cnt2&1) ans+=s[i];
                  else ans+=t[i];
              }
          }
          cout<<ans<<endl;
      }
      return 0;
  }
  /*
  
  */
  