#include <bits/stdc++.h>
using namespace std;
const int N = 85;
char get(char a, char b) {
  for (char i = 'a'; i <= 'z'; i++) {
    if (a != i && b != i) return i;
  }
}
int main() {
  int n, t, same = 0;
  string a, b, c;
  t = n - t;
  cin >> n >> t >> a >> b;
  c = a;
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i]) same++;
  }
  t = n - t;
  if ((t - same) * 2 + same > n) {
    cout << -1;
    return 0;
  }
  int x = t, y = t;
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i]) {
      if (x > 0 && y > 0)
        x--, y--;
      else {
        c[i] = get(a[i], b[i]);
      }
    }
  }
  for (int i = 0; i < n; i++) {
    if (a[i] != b[i]) {
      if (x) {
        x--;
        c[i] = a[i];
      } else if (y) {
        y--;
        c[i] = b[i];
      } else
        c[i] = get(a[i], b[i]);
    }
  }
  cout << c;
  return 0;
}
