#include <bits/stdc++.h>
using namespace std;
int n, t, k, x;
string s1, s2, s3;
int main() {
  cin >> n >> t >> s1 >> s2;
  if (t == 0) {
    if (s1 == s2)
      cout << s1;
    else
      cout << -1;
    return 0;
  }
  k = 0;
  for (int i = 0; i < n; i++)
    if (s1[i] != s2[i]) k++;
  if (k <= t) {
    x = t - k;
    s3 = "";
    for (int i = 0; i < n; i++)
      if ((s1[i] == s2[i] && x > 0) || s1[i] != s2[i]) {
        for (char c = 'a'; c <= 'z'; c++)
          if (c != s1[i] && c != s2[i]) {
            s3 += c;
            break;
          }
        if (s1[i] == s2[i]) x--;
      } else
        s3 += s1[i];
    cout << s3;
  } else {
    if (t * 2 < k) {
      cout << -1;
      return 0;
    }
    k = n - k;
    x = k = n - t - k;
    s3 = "";
    for (int i = 0; i < n; i++)
      if (s1[i] == s2[i])
        s3 += s1[i];
      else if (x > 0) {
        s3 += s1[i];
        x--;
      } else if (k > 0) {
        s3 += s2[i];
        k--;
      } else
        for (char c = 'a'; c <= 'z'; c++)
          if (c != s1[i] && c != s2[i]) {
            s3 += c;
            break;
          }
    cout << s3;
  }
  return 0;
}
