#include <bits/stdc++.h>
using namespace std;
int diff(string a, string b, int n) {
  int res = 0;
  for (int i = 0; i < n; i++)
    if (a[i] != b[i]) res++;
  return res;
}
char other(char a, char b) {
  for (char i = 'a'; i <= 'z'; i++)
    if (i != a && i != b) return i;
  return ' ';
}
string solve(string a, string b, int n, int t) {
  string res = "";
  for (int i = 0; i < n; i++) res += other(a[i], b[i]);
  vector<int> e, d;
  for (int i = 0; i < n; i++)
    if (a[i] == b[i])
      e.push_back(i);
    else
      d.push_back(i);
  int cur = n;
  for (int i = 0; i < e.size(); i++)
    if (cur > t) {
      res[e[i]] = a[e[i]];
      cur--;
    }
  for (int i = 0; i + 1 < d.size(); i += 2)
    if (cur > t) {
      res[d[i]] = a[d[i]];
      res[d[i + 1]] = b[d[i + 1]];
      cur--;
    }
  if (cur > t)
    return "-1";
  else
    return res;
}
int main() {
  int n, t;
  string a, b;
  cin >> n >> t >> a >> b;
  cout << solve(a, b, n, t);
  return 0;
}
