#include <bits/stdc++.h>
using namespace std;
const int ma = 1e5 + 5;
int n, t;
string a, b, ans;
bool vis[ma];
int main() {
  cin >> n >> t;
  cin >> a >> b;
  int same = n - t, cur = 0;
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i] && cur < same) {
      cur++;
      vis[i] = true;
    }
  }
  int x[2] = {cur, cur};
  for (int i = 0; i < n; i++) {
    if (vis[i]) {
      ans += a[i];
    } else {
      if (x[0] < same) {
        x[0]++;
        ans += a[i];
      } else {
        if (x[1] < same) {
          x[1]++;
          ans += b[i];
        } else {
          for (int j = 0; j < 26; j++) {
            if (a[i] - 'a' != j && b[i] - 'a' != j) {
              ans += 'a' + j;
              break;
            }
          }
        }
      }
    }
  }
  if (x[0] == same && x[1] == same) {
    cout << ans << endl;
  } else
    cout << -1 << endl;
  return 0;
}
