#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t;
  cin >> n >> t;
  string s1, s2, s3;
  cin >> s1 >> s2;
  s3 = s1;
  int t1 = t, t2 = t;
  for (int i = 0; i < n; i++) {
    if (s1[i] != s2[i]) {
      s3[i] = s2[i];
      t1--;
    }
    if (t1 == 0) {
      break;
    }
  }
  for (int i = 0; i < n; i++) {
    if (s3[i] != s2[i]) {
      t2--;
    }
  }
  for (int i = 0; i < n; i++) {
    if (t2 == 0) {
      break;
    }
    if (s3[i] == s2[i] && s3[i] != s1[i]) {
      for (int j = 0; j < 26; j++) {
        if ('a' + j != s2[i] && 'a' + j != s1[i]) {
          s3[i] = 'a' + j;
          t2--;
          break;
        }
      }
    }
  }
  for (int i = 0; i < n; i++) {
    if (t1 == 0) {
      break;
    }
    if (s3[i] == s1[i] && s3[i] == s2[i]) {
      for (int j = 0; j < 26; j++) {
        if ('a' + j != s2[i] && 'a' + j != s1[i]) {
          s3[i] = 'a' + j;
          t1--;
          t2--;
          break;
        }
      }
    }
  }
  if (t1 != t2 || t1 != 0 || t2 != 0) {
    cout << -1 << endl;
    return 0;
  }
  cout << s3 << endl;
  return 0;
}
