#include <bits/stdc++.h>
inline int Rand() { return (rand() << 16) | rand(); }
using namespace std;
const int N = 100001;
int n, m, kol, cmp, cmp2;
string s, ss, ans;
int sz, kol2;
bool ok;
int pos;
char other(char x, char y) {
  for (int i = 0; i < 26; i++) {
    if (char(i + 97) != x && char(i + 97) != y) return char(i + 97);
  }
}
int main() {
  srand(time(NULL));
  scanf("%d %d", &n, &m);
  cin >> s;
  m = n - m;
  cin >> ss;
  for (int i = 0; i < n; i++)
    if (s[i] == ss[i]) kol++;
  ans = s;
  if (n - kol < (m - kol) * 2) cout << -1, exit(0);
  kol = m - kol;
  for (int i = 0; i < n; i++) {
    if (s[i] == ss[i]) {
      if (m) {
        m--;
        ans[i] = s[i];
      } else {
        ans[i] = other(s[i], ss[i]);
      }
    } else if (kol > 0) {
      if (!ok)
        ans[i] = s[i], ok = true;
      else
        ans[i] = ss[i], ok = false, kol--;
    } else {
      ans[i] = other(s[i], ss[i]);
    }
  }
  cout << ans;
  return 0;
}
