#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t, cnt = 0, cntt, cntt1, cntt2, c1 = 0, c2 = 0, prev = 1;
  string s1, s2, s3;
  cin >> n >> t >> s1 >> s2;
  for (int i = 0; i < n; i++) {
    if (s1[i] == s2[i]) cnt++;
    s3 += "0";
  }
  if (t >= (n - cnt)) {
    cntt = cnt;
    for (int i = 0; i < n; i++) {
      if (s1[i] == s2[i] && (n - cntt) < t) {
        if (s1[i] + 1 <= 122)
          s3[i] = s1[i] + 1;
        else
          s3[i] = s1[i] - 1;
        cntt--;
      } else if (s1[i] != s2[i]) {
        if (s1[i] + 1 != s2[i] && s1[i] + 1 <= 122)
          s3[i] = s1[i] + 1;
        else if (s1[i] - 1 != s2[i] && s1[i] - 1 >= 97)
          s3[i] = s1[i] - 1;
        else if (s2[i] + 1 != s1[i] && s2[i] + 1 <= 122)
          s3[i] = s2[i] + 1;
        else
          s3[i] = s2[i] - 1;
      } else
        s3[i] = s2[i];
    }
  } else {
    cntt1 = (cntt2 = cnt);
    for (int i = 0; i < n; i++) {
      if (s1[i] != s2[i] && prev == 1 && (n - cntt2) > t) {
        s3[i] = s2[i];
        prev = 2;
        cntt2++;
      } else if (s1[i] != s2[i] && prev == 2 && (n - cntt1) > t) {
        s3[i] = s1[i];
        prev = 1;
        cntt1++;
      } else if (s1[i] == s2[i]) {
        s3[i] = s2[i];
      } else {
        if (s1[i] + 1 != s2[i] && s1[i] + 1 <= 122)
          s3[i] = s1[i] + 1;
        else if (s1[i] - 1 != s2[i] && s1[i] - 1 >= 97)
          s3[i] = s1[i] - 1;
        else if (s2[i] + 1 != s1[i] && s2[i] + 1 <= 122)
          s3[i] = s2[i] + 1;
        else
          s3[i] = s2[i] - 1;
      }
    }
  }
  for (int i = 0; i < n; i++) {
    if (s3[i] != s1[i]) c1++;
    if (s3[i] != s2[i]) c2++;
    if (s3[i] == '0') {
      cout << "-1" << endl;
      return 0;
    }
  }
  if (c1 != t || c2 != t) {
    cout << "-1" << endl;
    return 0;
  }
  cout << s3 << endl;
  return 0;
}
