#include <bits/stdc++.h>
using namespace std;
char output[100005];
char neither(char a, char b) {
  char output = 'a';
  while (output == a or output == b) output++;
  return output;
}
int main() {
  int n, t, mutuals = 0;
  string s1, s2;
  cin >> n >> t >> s1 >> s2;
  for (int i = 0; i < n; i++)
    if (s1[i] == s2[i]) mutuals++;
  if (t >= n - mutuals) {
    for (int i = 0; i < n and t != 0; i++) {
      if (s1[i] == s2[i]) continue;
      output[i] = neither(s1[i], s2[i]);
      t--;
    }
    for (int i = 0; i < n; i++) {
      if (s1[i] != s2[i]) continue;
      if (t == 0)
        output[i] = s1[i];
      else {
        output[i] = neither(s1[i], s1[i]);
        t--;
      }
    }
  } else if (t >= (n - mutuals) / 2) {
    int switcher = 0, las = 0;
    for (int i = 0; i < n; i++) {
      if (s1[i] == s2[i]) continue;
      if (switcher == 0)
        output[i] = s1[i];
      else
        output[i] = s2[i];
      switcher = (switcher + 1) % 2;
      las = i;
    }
    t -= (n - mutuals) / 2;
    if ((n - mutuals) % 2 == 1) {
      output[las] = neither(s1[las], s2[las]);
      t--;
    }
    for (int i = 0; i < n; i++) {
      if (s1[i] != s2[i]) continue;
      if (t == 0)
        output[i] = s1[i];
      else {
        output[i] = neither(s1[i], s1[i]);
        t--;
      }
    }
    if (t > 0) {
      switcher = 0;
      for (int i = 0; i < n and t != 0; i++) {
        if (s1[i] == s2[i]) continue;
        output[i] = neither(s1[i], s2[i]);
        switcher++;
        if (switcher == 2) {
          switcher = 0;
          t--;
        }
      }
    }
  } else
    t = -1;
  if (t < 0)
    cout << -1;
  else
    cout << output;
  return 0;
}
