#include <bits/stdc++.h>
using namespace std;
char getOther(const char c1, const char c2) {
  for (char i = 97; i < 100; ++i) {
    if (i != c1 && i != c2) return i;
  }
  return 'z';
}
int main() {
  int n, t;
  string s1, s2;
  cin >> n >> t >> s1 >> s2;
  bool e = false;
  string ret(s1.length(), 'a');
  bool f = true;
  if (!t && s1 == s2) {
    ret = s1;
    e = true;
  }
  for (size_t i = 0; i < s1.length() && t; ++i) {
    if (s1[i] == s2[i])
      ret[i] = s1[i];
    else {
      ret[i] = f ? s1[i] : s2[i];
      if (!f) --t;
      f = !f;
    }
    if (!t) {
      e = true;
      for (size_t j = i + 1; j < s1.length(); ++j) {
        if (s1[j] != s2[j]) {
          e = false;
          break;
        }
        ret[j] = s1[j];
      }
      if (e) break;
    }
  }
  for (int i = s1.length() - 1; i >= 0 && t; --i) {
    if (s1[i] != s2[i]) {
      ret[i] = getOther(s1[i], s2[i]);
      if (!f) --t;
      f = !f;
    }
    if (!t) {
      e = true;
      break;
    }
  }
  for (size_t i = 0; i < s1.length() && t; ++i) {
    if (s1[i] == s2[i]) {
      ret[i] = getOther(s1[i], s2[i]);
      --t;
    }
    if (!t) {
      e = true;
      break;
    }
  }
  if (e)
    cout << ret;
  else
    cout << -1;
  return 0;
}
