#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 10;
char s1[maxn], s2[maxn];
int n, t;
int main() {
  scanf("%d%d", &n, &t);
  scanf("%s%s", s1, s2);
  int len = strlen(s1);
  int dif = 0;
  for (int i = 0; i < len; i++)
    if (s1[i] != s2[i]) dif++;
  if (t >= dif) {
    int cnt = t - dif;
    for (int i = 0; i < len; i++) {
      if (s1[i] == s2[i]) {
        if (cnt) {
          for (int j = 0; j < 26; j++)
            if (j + 'a' != s1[i]) {
              putchar(j + 'a');
              break;
            }
          cnt--;
        } else
          putchar(s1[i]);
      } else {
        for (int j = 0; j < 26; j++)
          if (j + 'a' != s1[i] && j + 'a' != s2[i]) {
            putchar(j + 'a');
            break;
          }
      }
    }
  } else {
    int y = dif - t, x = 2 * t - dif;
    if (2 * y > dif)
      printf("-1\n");
    else {
      for (int i = 0; i < len; i++) {
        if (s1[i] == s2[i])
          putchar(s1[i]);
        else {
          if (x) {
            for (int j = 0; j < 26; j++)
              if (j + 'a' != s1[i] && j + 'a' != s2[i]) {
                putchar(j + 'a');
                break;
              }
            x--;
          } else if (y) {
            putchar(s1[i]);
            y--;
          } else {
            putchar(s2[i]);
          }
        }
      }
    }
  }
  return 0;
}
