#include <bits/stdc++.h>
using namespace std;
int n, t;
string s1, s2, s3;
int getDiff() {
  int nr = 0;
  for (int i = 0; i < n; i++) {
    if (s1[i] != s2[i]) nr++;
  }
  return nr;
}
char getOther(char a, char b) {
  if (a == 'a' || b == 'a') {
    if (a == 'b' || b == 'b') {
      return 'c';
    }
    return 'b';
  }
  return 'a';
}
int main() {
  cin >> n >> t >> s1 >> s2;
  int k = getDiff();
  if (t < (k + 1) / 2) {
    cout << "-1\n";
    return 0;
  }
  if (k <= t) {
    int modif = t - k;
    for (int i = 0; i < n; i++) {
      if (s1[i] != s2[i]) {
        s3 += getOther(s1[i], s2[i]);
      } else {
        if (modif > 0) {
          s3 += getOther(s1[i], s2[i]);
          modif--;
        } else {
          s3 += s1[i];
        }
      }
    }
  }
  if (k > t) {
    int modif = k - t;
    bool first = true;
    for (int i = 0; i < n; i++) {
      if (s1[i] == s2[i]) {
        s3 += s1[i];
      } else {
        if (first) {
          s3 += s1[i];
          modif--;
          if (modif == 0) {
            first = false;
            modif = k - t;
          }
        } else {
          if (modif > 0) {
            s3 += s2[i];
            modif--;
          } else {
            s3 += getOther(s1[i], s2[i]);
          }
        }
      }
    }
  }
  cout << s3 << "\n";
  return 0;
}
