#include <bits/stdc++.h>
using namespace std;
char find(char a, char b) {
  for (char f = 'a';; f++)
    if (f != a && f != b) return f;
}
int main() {
  int n;
  int t;
  cin >> n >> t;
  char* s1 = new char[n];
  char* s2 = new char[n];
  char* s3 = new char[n];
  cin >> s1;
  cin >> s2;
  int diff = 0;
  int d1 = 0, d2 = 0;
  for (int i = 0; i < n; i++)
    if (s1[i] != s2[i]) diff++;
  int same = n - diff;
  int me = n - t;
  if (me <= same) {
    for (int i = 0; i < n; i++) {
      if (s1[i] == s2[i] && me > 0) {
        s3[i] = s1[i];
        me--;
      } else
        s3[i] = find(s1[i], s2[i]);
    }
  } else {
    if (same + (n - same) / 2 >= me) {
      int m1 = 0, m2 = 0;
      int mx = me - same;
      for (int i = 0; i < n; i++) {
        if (s1[i] == s2[i])
          s3[i] = s1[i];
        else {
          if (m1 < mx) {
            s3[i] = s1[i];
            m1++;
          } else if (m2 < mx) {
            s3[i] = s2[i];
            m2++;
          } else
            s3[i] = find(s1[i], s2[i]);
        }
      }
    } else {
      cout << -1;
      return 0;
    }
  }
  for (int i = 0; i < n; i++) cout << s3[i];
  return 0;
}
