#include <bits/stdc++.h>
int n, t, diff = 0;
char s1[100001], s2[100001], s3[100001];
int main() {
  scanf("%d %d\n", &n, &t);
  gets(s1);
  gets(s2);
  for (int i = 0; i < n; i++)
    if (s1[i] != s2[i]) diff++;
  if (diff > t * 2)
    printf("-1\n");
  else {
    if (t >= diff) {
      int cnt = t - diff;
      for (int i = 0; i < n; i++) {
        if (s1[i] != s2[i] || cnt) {
          if (s1[i] == s2[i]) cnt--;
          char c = 'a';
          while (s1[i] == c || s2[i] == c) c++;
          s3[i] = c;
        } else
          s3[i] = s1[i];
      }
    } else {
      int both = t * 2 - diff, each = (t - both) * 2;
      for (int i = 0; i < n; i++) {
        if (s1[i] != s2[i]) {
          if (both > 0) {
            char c = 'a';
            while (s1[i] == c || s2[i] == c) c++;
            s3[i] = c;
            both--;
          } else {
            if (each % 2)
              s3[i] = s1[i];
            else
              s3[i] = s2[i];
            each--;
          }
        } else
          s3[i] = s1[i];
      }
    }
    puts(s3);
  }
  return 0;
}
