#include <bits/stdc++.h>
using namespace std;
int n;
char s1[111111];
char s2[111111];
char res[111111];
int t, k, eq;
char gd(char u, char v) {
  int i;
  for (i = 0; i <= 25; i++) {
    if ('a' + i != u && 'a' + i != v) {
      return 'a' + i;
    }
  }
}
int main(void) {
  cin >> n >> t;
  k = n - t;
  scanf("%s", &s1[1]);
  scanf("%s", &s2[1]);
  int i;
  for (i = 1; i <= n; i++) {
    if (s1[i] == s2[i]) {
      eq++;
    }
  }
  if (eq >= k) {
    int cur = 0;
    for (i = 1; i <= n; i++) {
      if (s1[i] != s2[i] || cur >= k) {
        res[i] = gd(s1[i], s2[i]);
      } else {
        cur++;
        res[i] = s1[i];
      }
    }
    for (i = 1; i <= n; i++) {
      printf("%c", res[i]);
    }
    return 0;
  } else {
    if (n - eq < 2 * (k - eq)) {
      cout << -1;
      return 0;
    }
    int cur1 = 0, cur2 = 0;
    for (i = 1; i <= n; i++) {
      if (s1[i] == s2[i]) {
        res[i] = s1[i];
      } else if (cur1 < k - eq) {
        cur1++;
        res[i] = s1[i];
      } else if (cur2 < k - eq) {
        cur2++;
        res[i] = s2[i];
      } else {
        res[i] = gd(s1[i], s2[i]);
      }
    }
    for (i = 1; i <= n; i++) {
      printf("%c", res[i]);
    }
  }
}
