#include <bits/stdc++.h>
using namespace std;
int main() {
  long n, t;
  char s1[100001], s2[100001], answer[100001];
  cin >> n >> t;
  n--;
  cin >> s1 >> s2;
  long dif = 0;
  for (long i = 0; i <= n; i++)
    if (s1[i] != s2[i]) dif++;
  if (dif > t) {
    long n1 = dif - t;
    long n2 = n1;
    if (n1 + n2 > dif) {
      cout << "-1\n";
      return 0;
    }
    for (long i = 0; i <= n; i++) {
      if (s1[i] == s2[i])
        answer[i] = s1[i];
      else {
        if (n1) {
          answer[i] = s1[i];
          n1--;
        } else if (n2) {
          answer[i] = s2[i];
          n2--;
        } else {
          char c;
          for (long j = 0; j <= 25; j++) {
            c = 'a' + j;
            if (c != s1[i] && c != s2[i]) break;
          }
          answer[i] = c;
        }
      }
    }
  } else {
    long NN = t - dif;
    for (long i = 0; i <= n; i++) {
      if (s1[i] == s2[i]) {
        if (NN) {
          if (s1[i] == 'z')
            answer[i] = 'x';
          else
            answer[i] = s1[i] + 1;
          NN--;
        } else
          answer[i] = s1[i];
      } else {
        char c;
        for (long j = 0; j <= 25; j++) {
          c = 'a' + j;
          if (c != s1[i] && c != s2[i]) break;
        }
        answer[i] = c;
      }
    }
  }
  cout << answer;
  return 0;
}
