#include <bits/stdc++.h>
using namespace std;
template <typename T>
T logPow(T x, long long y) {
  T temp;
  if (y == 0) {
    return 1LL;
  }
  temp = logPow(x, y / 2);
  if (y % 2 == 0) {
    return temp * temp;
  } else {
    if (y > 0) {
      return x * temp * temp;
    } else {
      return (temp * temp) / x;
    }
  }
}
long long modExp(long long base, long long exp) {
  long long res = 1;
  base = (base % 1000000007);
  while (exp) {
    if (exp & 1) {
      res = (res * base) % 1000000007;
    }
    exp >>= 1;
    base = (base * base) % 1000000007;
  }
  return res;
}
bool primes[32000];
vector<int> primeList;
void sieve() {
  fill(primes, primes + 32000, true);
  for (int i = 2; i <= sqrt(32000); ++i) {
    if (primes[i]) {
      for (int j = i * i; j <= 32000; j += i) {
        primes[j] = false;
      }
    }
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n, t;
  cin >> n >> t;
  string s1, s2;
  cin >> s1 >> s2;
  int cnt = 0;
  for (int i = 0; i < n; ++i) {
    if (s1[i] == s2[i]) {
      cnt++;
    }
  }
  int diff = n - cnt;
  int mn = ceil(diff / 2.0);
  if (t < mn) {
    cout << -1 << endl;
    return 0;
  }
  string req = "";
  int diff1 = 0, diff2 = 0, last = 0;
  for (int i = 0; i < n; ++i) {
    if (s1[i] != s2[i]) {
      if (last == 0 && diff1 < diff / 2) {
        req += s1[i];
        diff1++;
        last = !last;
      } else if (last == 1 && diff2 < diff / 2) {
        req += s2[i];
        diff2++;
        last = !last;
      } else {
        for (char c = 'a'; c <= 'z'; ++c) {
          if (c != s1[i] && c != s2[i]) {
            req += c;
            break;
          }
        }
      }
    } else {
      req += s1[i];
    }
  }
  int chngs = mn;
  int remChngs = t - chngs;
  if (remChngs == 0) {
    cout << req << endl;
    return 0;
  }
  for (int i = 0; i < n; ++i) {
    if (s1[i] == s2[i]) {
      for (char c = 'a'; c <= 'z'; ++c) {
        if (c != s1[i]) {
          req[i] = c;
          break;
        }
      }
      remChngs--;
      if (remChngs == 0) {
        cout << req << endl;
        return 0;
      }
    }
  }
  last = 0;
  for (int i = 0; i < n; ++i) {
    if (s1[i] != s2[i]) {
      for (char c = 'a'; c <= 'z'; ++c) {
        if (c != s1[i] && c != s2[i]) {
          req[i] = c;
          break;
        }
      }
      last = !last;
      if (!last) {
        remChngs--;
        if (remChngs == 0) {
          cout << req << endl;
          return 0;
        }
      }
    }
  }
  return 0;
}
