#include <bits/stdc++.h>
using namespace std;
const int MOD = 1000000007;
template <class T>
typename T::value_type arr_sum(const T& v, int n) {
  typename T::value_type sum = 0;
  for (int i = (0); i < (n); ++i) sum += v[i];
  return sum;
}
struct Sync_stdio {
  Sync_stdio() {
    cin.tie(NULL);
    ios_base::sync_with_stdio(false);
  }
} _sync_stdio;
int main() {
  int n, t;
  cin >> n >> t;
  t = n - t;
  string s1, s2;
  cin >> s1 >> s2;
  string s3(n, '?');
  for (int i = (0); i < (n); ++i) {
    if (t > 0 && s1[i] == s2[i]) {
      s3[i] = s1[i];
      --t;
    }
  }
  int i = 0;
  int j = 0;
  while (t > 0) {
    while (i < n && s3[i] != '?') {
      ++i;
    }
    j = i + 1;
    while (j < n && s3[j] != '?') {
      ++j;
    }
    if (i == n || j == n) {
      break;
    }
    s3[i] = s1[i];
    s3[j] = s2[j];
    --t;
  }
  if (t != 0) {
    cout << -1;
    return 0;
  }
  for (int i = (0); i < (n); ++i) {
    if (s3[i] == '?') {
      for (int j = ('a'); j < ('z' + 1); ++j) {
        if (s1[i] != j && s2[i] != j) {
          s3[i] = j;
          break;
        }
      }
    }
  }
  cout << s3;
}
