#include <bits/stdc++.h>
using namespace std;
long long int gcd(long long int a, long long int b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long int lcm(long long int a, long long int b) {
  return a * b / gcd(a, b);
}
long long int fexp(long long int a, long long int b) {
  long long int ans = 1;
  while (b) {
    if (b & 1) ans = ans * a % 1000000007;
    b /= 2;
    a = a * a % 1000000007;
  }
  return ans;
}
long long int inverse(long long int a, long long int p) {
  return fexp(a, p - 2);
}
void ingraph(vector<vector<long long int> >& graph, long long int m) {
  long long int x, y;
  for (long long int i = 0; i < m; i++) {
    cin >> x >> y;
    x--, y--;
    graph[x].push_back(y);
    graph[y].push_back(x);
  }
}
char getran(char a, char b) {
  for (char x = 'a'; x < 'z'; x++) {
    if (x != a && x != b) return x;
  }
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int t;
  t = 1;
  while (t--) {
    long long int n, t;
    cin >> n >> t;
    string x, y;
    cin >> x >> y;
    vector<long long int> diff, sam;
    for (long long int i = 0; i < n; i++) {
      if (x[i] == y[i])
        sam.push_back(i);
      else
        diff.push_back(i);
    }
    if (diff.size() > 2 * t)
      cout << -1 << endl;
    else {
      char ans[n];
      if (diff.size() <= t) {
        for (long long int i = 0; i < diff.size(); i++) {
          ans[diff[i]] = getran(x[diff[i]], y[diff[i]]);
          t--;
        }
        long long int j = 0;
        while (t) {
          ans[sam[j]] = getran('a', x[sam[j]]);
          t--;
          j++;
        }
        for (long long int i = j; i < sam.size(); i++) ans[sam[i]] = x[sam[i]];
      } else {
        long long int j = 0;
        for (long long int i = 0; i < diff.size() - t; i++) {
          ans[diff[j]] = y[diff[j]];
          j++;
        }
        for (long long int i = 0; i < diff.size() - t; i++) {
          ans[diff[j]] = x[diff[j]];
          j++;
        }
        for (long long int i = 0; i < 2 * t - diff.size(); i++) {
          ans[diff[j]] = getran(x[diff[j]], y[diff[j]]);
          j++;
        }
        for (long long int i = 0; i < sam.size(); i++) ans[sam[i]] = x[sam[i]];
      }
      for (long long int i = 0; i < n; i++) cout << ans[i];
    }
  }
}
