#include <bits/stdc++.h>
using namespace std;
int n, t;
string str1, str2;
int main() {
  scanf("%d %d", &n, &t);
  cin >> str1 >> str2;
  int l1 = n - t, l2 = n - t;
  string ans;
  ans.resize(n);
  for (int i = 0; i < n; i++) {
    if (l1 == 0 && l2 == 0) break;
    if (str1[i] == str2[i]) {
      ans[i] = str1[i];
      l1--;
      l2--;
    }
  }
  if (l1 == 0 && l2 == 0) {
    for (int i = 0; i < n; i++) {
      if (ans[i] >= 'a' && ans[i] <= 'z') continue;
      for (int j = 'a'; j <= 'z'; j++) {
        if (j != str1[i] && j != str2[i]) {
          ans[i] = j;
          break;
        }
      }
    }
  } else {
    int state = 0;
    for (int i = 0; i < n; i++) {
      if (l1 == 0 && l2 == 0) {
        if (ans[i] >= 'a' && ans[i] <= 'z') continue;
        for (int j = 'a'; j <= 'z'; j++) {
          if (j != str1[i] && j != str2[i]) {
            ans[i] = j;
            break;
          }
        }
      } else {
        if (ans[i] >= 'a' && ans[i] <= 'z') continue;
        if (state == 0) {
          l1--;
          ans[i] = str1[i];
          state = 1;
        } else {
          l2--;
          ans[i] = str2[i];
          state = 0;
        }
      }
    }
  }
  if (l1 > 0 || l2 > 0)
    printf("-1\n");
  else
    cout << ans << endl;
}
