#include <bits/stdc++.h>
using namespace std;
char dif(char c1) {
  if (c1 == 'a') return 'b';
  return 'a';
}
char dif2(char c1, char c2) {
  if (c1 != 'a' && c2 != 'a') return 'a';
  if (c1 != 'b' && c2 != 'b') return 'b';
  return 'c';
}
int main() {
  int n, t;
  cin >> n >> t;
  string s1, s2;
  cin >> s1 >> s2;
  vector<int> com;
  vector<int> nocom;
  for (long long i = 0; i < (n); i++)
    if (s1[i] == s2[i])
      com.push_back(i);
    else
      nocom.push_back(i);
  int comsz = com.size();
  int nocomsz = nocom.size();
  if (!(2 * t >= nocomsz)) {
    cout << -1 << endl;
    return 0;
  }
  string s3 = s1;
  if (nocomsz & 1) {
    int ind = nocom[nocomsz - 1];
    s3[ind] = dif2(s1[ind], s2[ind]);
    t--;
  }
  for (int i = 0; (i + 1 < nocomsz) && t > 0; i += 2) {
    int ind1 = nocom[i];
    s3[ind1] = s2[ind1];
    int ind2 = nocom[i + 1];
    s3[ind2] = s1[ind2];
    t--;
  }
  for (int i = 0; i < comsz && t > 0; i++) {
    int ind = com[i];
    s3[ind] = dif(s1[ind]);
    t--;
  }
  for (int i = 0; (i + 1 < nocomsz) && t > 0; i += 2) {
    int ind1 = nocom[i];
    int ind2 = nocom[i + 1];
    s3[ind1] = dif2(s1[ind1], s2[ind1]);
    s3[ind2] = dif2(s1[ind2], s2[ind2]);
    t--;
  }
  cout << s3 << endl;
}
