#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, t, t1 = 0, t2 = 0, k = -1;
  string s1, s2, s3;
  cin >> n >> t >> s1 >> s2;
  s3.resize(n);
  for (int i = 0; i < n; i++) {
    if (s1[i] != s2[i]) {
      if (t2 >= t1) {
        t1++;
        s3[i] = s2[i];
      } else if (t1 >= t2) {
        t2++;
        s3[i] = s1[i];
      }
      k = i;
    } else if (s1[i] == s2[i]) {
      s3[i] = s1[i];
    }
  }
  if (t1 > t2) {
    if (s1[k] != 'a' && s2[k] != 'a') {
      s3[k] = 'a';
    } else if (s1[k] != 'b' && s2[k] != 'b') {
      s3[k] = 'b';
    } else if (s1[k] != 'c' && s2[k] != 'c') {
      s3[k] = 'c';
    }
    t2++;
  }
  if (t1 > t) {
    cout << -1 << endl;
  } else {
    for (int i = 0; i < n; i++) {
      if (s1[i] == s2[i] && t1 < t) {
        t1++;
        t2++;
        if (s1[i] == 'a') {
          s3[i] = 'b';
        } else {
          s3[i] = 'a';
        }
      }
    }
    for (int i = 0; i < n; i++) {
      if (s1[i] == s3[i] && t1 < t) {
        if (s1[i] != 'a' && s2[i] != 'a') {
          s3[i] = 'a';
        } else if (s1[i] != 'b' && s2[i] != 'b') {
          s3[i] = 'b';
        } else if (s1[i] != 'c' && s2[i] != 'c') {
          s3[i] = 'c';
        }
        t1++;
      } else if (s2[i] == s3[i] && t2 < t) {
        if (s1[i] != 'a' && s2[i] != 'a') {
          s3[i] = 'a';
        } else if (s1[i] != 'b' && s2[i] != 'b') {
          s3[i] = 'b';
        } else if (s1[i] != 'c' && s2[i] != 'c') {
          s3[i] = 'c';
        }
        t2++;
      }
    }
    cout << s3 << endl;
  }
  return 0;
}
