Problem: Marina loves strings of the same length and Vasya loves when there is a third string, different from them in exactly t characters. Help Vasya find at least one such string.

More formally, you are given two strings s1, s2 of length n and number t. Let's denote as f(a, b) the number of characters in which strings a and b are different. Then your task will be to find any string s3 of length n, such that f(s1, s3) = f(s2, s3) = t. If there is no such string, print  - 1.

Input

The first line contains two integers n and t (1 ≤ n ≤ 105, 0 ≤ t ≤ n).

The second line contains string s1 of length n, consisting of lowercase English letters.

The third line contain string s2 of length n, consisting of lowercase English letters.

Output

Print a string of length n, differing from string s1 and from s2 in exactly t characters. Your string should consist only from lowercase English letters. If such string doesn't exist, print -1.

Examples

Input

3 2
abc
xyc


Output

ayd

Input

1 0
c
b


Output

-1

Solution (in CPP):

Solution:
#include <bits/stdc++.h>
using namespace std;
void __print(int x) { cout << x; }
void __print(long x) { cout << x; }
void __print(long long x) { cout << x; }
void __print(unsigned x) { cout << x; }
void __print(unsigned long x) { cout << x; }
void __print(unsigned long long x) { cout << x; }
void __print(float x) { cout << x; }
void __print(double x) { cout << x; }
void __print(long double x) { cout << x; }
void __print(char x) { cout << '\'' << x << '\''; }
void __print(const char *x) { cout << '\"' << x << '\"'; }
void __print(const string &x) { cout << '\"' << x << '\"'; }
void __print(bool x) { cout << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cout << '{';
  __print(x.first);
  cout << ',';
  __print(x.second);
  cout << '}';
}
template <typename T>
void __print(const T &x) {
  int f = 0;
  cout << '{';
  for (auto &i : x) cout << (f++ ? "," : ""), __print(i);
  cout << "}";
}
void _print() { cout << "\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cout << ", ";
  _print(v...);
}
const int mod = 1e9 + 7, mxn = 1e5 + 1;
int solve() {
  int n, k, tmp;
  cin >> n >> k;
  tmp = k;
  k = n - k;
  string a, b, ans;
  cin >> a >> b;
  for (long long i = (0); i < (n); ++i) ans += "@";
  vector<int> v;
  for (long long i = (0); i < (n); ++i) {
    if (a[i] == b[i]) v.push_back(i);
  }
  int x = v.size();
  if (x + (n - x) / 2 < k) return cout << "-1\n", 0;
  for (long long i = (0); i < (min(k, (int)v.size())); ++i) ans[v[i]] = a[v[i]];
  int cnt = min((int)v.size(), k);
  k -= cnt;
  if (k) {
    int kk = k;
    int i = 0;
    while (k) {
      if (i == n) break;
      if (ans[i] != '@') {
        i++;
        continue;
      }
      ans[i] = a[i];
      i++;
      k--;
    }
    while (kk) {
      if (i == n) break;
      if (ans[i] != '@') {
        i++;
        continue;
      }
      ans[i] = b[i];
      i++;
      kk--;
    }
  }
  for (long long i = (0); i < (n); ++i) {
    if (ans[i] != '@') continue;
    char diff;
    for (long long j = (0); j < (26); ++j) {
      diff = (char)('a' + j);
      if (diff == a[i] or diff == b[i]) continue;
      break;
    }
    ans[i] = diff;
  }
  cout << ans << "\n";
  return 0;
}
int main() {
  ios_base::sync_with_stdio(NULL);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}