#include <bits/stdc++.h>
using namespace std;
const int inf = 0x3f;
const int INF = 0x3f3f3f3f;
int n, t;
string a, b;
int main() {
  while (cin >> n >> t) {
    cin >> a >> b;
    int same = 0, diff = 0;
    for (int i = 0; i < n; i++) {
      if (a[i] == b[i])
        same++;
      else
        diff++;
    }
    if (same + diff == t) {
      for (int i = 0; i < n; i++) {
        for (int j = 0; j < 26; j++) {
          char c = (char)(j + 97);
          if (c != a[i] && c != b[i]) {
            cout << c;
            break;
          }
        }
      }
    } else {
      if (diff <= t) {
        int remain = t - diff;
        char c;
        for (int i = 0; i < n; i++) {
          for (int j = 0; j < 26; j++) {
            c = (char)(j + 97);
            if (c != a[i] && c != b[i]) break;
          }
          if (a[i] != b[i])
            cout << c;
          else {
            if (remain) {
              remain--;
              cout << c;
            } else
              cout << a[i];
          }
        }
      } else {
        if ((diff + 1) / 2 > t)
          cout << -1;
        else {
          int flag = 0;
          int remain = 2 * diff - 2 * t;
          for (int i = 0; i < n; i++) {
            if (a[i] != b[i]) {
              if (remain) {
                if (flag)
                  cout << a[i];
                else
                  cout << b[i];
                flag = !flag;
                remain--;
              } else {
                for (int j = 0; j < 26; j++) {
                  char c = (char)(j + 97);
                  if (c != a[i] && c != b[i]) {
                    cout << c;
                    break;
                  }
                }
              }
            } else
              cout << a[i];
          }
        }
      }
    }
    cout << endl;
  }
  return 0;
}
