#include <bits/stdc++.h>
using namespace std;
char mex(char a, char b) {
  if (a > b) swap(a, b);
  if (a == 'a' && b == 'b')
    return 'c';
  else if (a == 'a')
    return 'b';
  return 'a';
}
int main() {
  int n, k;
  cin >> n >> k;
  string a, b;
  cin >> a >> b;
  int diff = 0;
  for (int i = 0; i < n; i++) diff += a[i] != b[i];
  if (diff > 2 * k) {
    cout << -1 << endl;
    return 0;
  }
  string s;
  if (diff <= k) {
    s = a;
    int ch = diff;
    for (int i = 0; i < n; i++) {
      if (a[i] != b[i] || ch < k) {
        s[i] = mex(a[i], b[i]);
        if (a[i] == b[i]) ch++;
      }
    }
  } else {
    s = a;
    int cha = 0, chb = 0;
    for (int i = 0; i < n; i++) {
      if (a[i] != b[i]) {
        if (k + cha < diff) {
          s[i] = a[i];
          cha++;
        } else if (k + chb < diff) {
          s[i] = b[i];
          chb++;
        } else
          s[i] = mex(a[i], b[i]);
      }
    }
    assert(k + cha == diff);
    assert(k + chb == diff);
  }
  cout << s << endl;
  return 0;
}
