#include <bits/stdc++.h>
int n, t, cnt, chk;
char a[100000], b[100000], c[100000];
int main() {
  scanf("%d %d\n", &n, &t);
  scanf("%s %s", a, b);
  for (int i = 0; i < n; i++)
    if (a[i] != b[i]) cnt++;
  if (cnt <= t) {
    for (int i = 0; i < n; i++)
      if (a[i] != b[i])
        for (int j = 'a'; j <= 'z'; j++)
          if (a[i] != j && b[i] != j) c[i] = j;
    for (int i = 0; i < n; i++) {
      if (c[i] != 0) continue;
      if (a[i] == b[i] && t > cnt) {
        cnt++;
        for (int j = 'a'; j <= 'z'; j++)
          if (a[i] != j) c[i] = j;
      } else
        c[i] = a[i];
    }
    for (int i = 0; i < n; i++) printf("%c", c[i]);
  } else if (cnt > t * 2)
    printf("-1");
  else {
    int flag = 0;
    for (int i = 0; i < n; i++) {
      if (flag == 0 && a[i] != b[i]) {
        c[i] = a[i];
        flag = 1;
      } else {
        if (a[i] == b[i]) continue;
        c[i] = b[i];
        flag = 0;
        cnt--;
      }
      if (cnt == t) break;
    }
    for (int i = 0; i < n; i++) {
      if (c[i] != 0) continue;
      if (a[i] != b[i])
        for (int j = 'a'; j <= 'z'; j++)
          if (a[i] != j && b[i] != j) c[i] = j;
    }
    for (int i = 0; i < n; i++) {
      if (c[i] != 0) continue;
      if (a[i] == b[i] && t > cnt) {
        cnt++;
        for (int j = 'a'; j <= 'z'; j++)
          if (a[i] != j) c[i] = j;
      } else
        c[i] = a[i];
    }
    for (int i = 0; i < n; i++) printf("%c", c[i]);
  }
  return 0;
}
