#include <bits/stdc++.h>
using namespace std;
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  cerr << name << " : " << arg1 << '\n';
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}
int inf = 0x3f3f3f3f;
long long infl = 0x3f3f3f3f3f3f3f3fLL;
long double infd = 1.0 / 0.0;
const long long MOD = 1e9 + 7;
const long double pi = 2 * acos(0.0);
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  ;
  int n, c;
  cin >> n >> c;
  string s, t;
  cin >> s >> t;
  if (c == 0) {
    if (s == t) return cout << s, 0;
    return cout << -1, 0;
  }
  int x = n - c;
  vector<int> id(n);
  int diff = 0;
  for (int i = 0; i < n; i++) {
    if (s[i] == t[i] && x) {
      x--;
      id[i] = 1;
    }
    if (s[i] != t[i]) diff++;
  }
  if (2 * x > diff) return cout << -1, 0;
  int last = n - 1;
  for (int i = 0; i < n; i++) {
    if (id[i] == -1) {
      cout << t[i];
      continue;
    }
    if (id[i]) {
      cout << s[i];
      continue;
    }
    if (!x) {
      char gg = 'a';
      if (s[i] == gg) gg++;
      if (t[i] == gg) gg++;
      if (s[i] == gg) gg++;
      cout << gg;
      continue;
    }
    cout << s[i];
    x--;
    while (last >= 0) {
      if (id[last]) {
        last--;
        continue;
      }
      id[last] = -1;
      last--;
      break;
    }
  }
  return 0;
}
