#include <bits/stdc++.h>
using namespace std;
const unsigned long long MOD = 1e9 + 7;
string s1, s2, s3;
long long n, t, p1 = 0, p2 = 0;
char foo(char ch1, char ch2) {
  if (('a' != ch1) && ('a' != ch2)) return 'a';
  if (('b' != ch1) && ('b' != ch2)) return 'b';
  if (('c' != ch1) && ('c' != ch2)) return 'c';
}
int main() {
  cin >> n >> t >> s1 >> s2;
  s3 = s1;
  for (int i = 0; i < n; i++) {
    if (s1[i] == s2[i]) {
      p1++;
      p2++;
    } else {
      if (p1 < p2) {
        p1++;
        s3[i] = s1[i];
      } else {
        p2++;
        s3[i] = s2[i];
      }
    }
  }
  if ((t < n - p1) || (t < n - p2)) {
    cout << -1;
    return 0;
  }
  for (int i = 0; i < n; i++) {
    if (s1[i] == s2[i]) {
      continue;
    } else {
      if ((s2[i] == s3[i]) && (n - p2 < t)) {
        s3[i] = foo(s1[i], s2[i]);
        p2--;
      } else {
        if ((s1[i] == s3[i]) && (n - p1 < t)) {
          s3[i] = foo(s1[i], s2[i]);
          p1--;
        }
      }
    }
  }
  for (int i = 0; i < n; i++) {
    if ((s2[i] == s3[i]) && (s1[i] == s3[i]) &&
        ((n - p1 < t) || (n - p2 < t))) {
      s3[i] = foo(s1[i], s2[i]);
      p2--;
      p1--;
    }
  }
  for (int i = 0; i < n; i++) {
    if ((n - p1 > t) && (s1[i] != s2[i]) && (s3[i] != s1[i]) &&
        (s3[i] != s2[i])) {
      s3[i] = s1[i];
      p1++;
    }
    if ((n - p2 > t) && (s1[i] != s2[i]) && (s3[i] != s1[i]) &&
        (s3[i] != s2[i])) {
      s3[i] = s2[i];
      p2++;
    }
  }
  if ((n - p1 != t) || (n - p2 != t)) {
    cout << -1;
    return 0;
  }
  cout << s3;
  return 0;
}
