#include <bits/stdc++.h>
using namespace std;
map<int, int> mp;
int main() {
  string s, s2, s3 = "";
  int n, t, i;
  cin >> n >> t >> s >> s2;
  int t1 = t, t2 = t;
  if (t1 == 0 && t2 == 0 && s[0] != s2[0]) {
    cout << "-1";
    return 0;
  }
  for (i = 0; i < n; i++) {
    if (t1 > 0 && s[i] != s2[i]) {
      s3 += s2[i];
      t1--;
    } else if (t2 > 0 && s[i] != s2[i]) {
      s3 += s[i];
      t2--;
    } else if (t1 == 0 && t2 == 0) {
      if (s[i] != s2[i]) {
        cout << "-1";
        return 0;
      }
    }
    if (s[i] == s2[i]) s3 += s[i];
  }
  for (i = 0; i < n; i++) {
    if (s[i] == s2[i] && t1 > 0 && t2 > 0) {
      t1--;
      t2--;
      if (s[i] != 'a')
        s3[i] = 'a';
      else
        s3[i] = 'b';
    } else if (s[i] != s2[i] && (t1 != 0 || t2 > 0)) {
      if (s[i] == s3[i])
        t1--;
      else
        t2--;
      if (s[i] != 'a' && s2[i] != 'a')
        s3[i] = 'a';
      else if (s[i] != 'b' && s2[i] != 'b')
        s3[i] = 'b';
      else
        s3[i] = 'c';
    }
  }
  cout << s3;
  return 0;
}
