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;
using namespace placeholders;
using LL = long long;
using ULL = unsigned long long;
using VI = vector<int>;
using VVI = vector<vector<int> >;
using VS = vector<string>;
using SS = stringstream;
using PII = pair<int, int>;
using VPII = vector<pair<int, int> >;
template <typename T = int>
using VT = vector<T>;
template <typename T = int>
using VVT = vector<vector<T> >;
template <typename T = int>
using LIM = numeric_limits<T>;
template <typename T>
inline istream &operator>>(istream &s, vector<T> &v) {
  for (T &t : v) {
    s >> t;
  }
  return s;
}
template <typename T>
inline ostream &operator<<(ostream &s, const vector<T> &v) {
  for (int i = 0; i < int(v.size()); ++i) {
    s << (" " + !i) << v[i];
  }
  return s;
}
template <typename T>
inline T fromString(const string &s) {
  T res;
  istringstream iss(s);
  iss >> res;
  return res;
};
template <typename T>
inline string toString(const T &a) {
  ostringstream oss;
  oss << a;
  return oss.str();
};
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int N, T;
  cin >> N >> T;
  string S1, S2;
  cin >> S1 >> S2;
  queue<int> mismatches;
  for (int i = (int)(0); i < (int)(N); ++i) {
    if (S1[i] != S2[i]) {
      mismatches.push(i);
    }
  }
  if (2 * T < ((int)(mismatches).size())) {
    cout << -1 << endl;
    return 0;
  }
  string res(N, '.');
  while (T < ((int)(mismatches).size())) {
    if (((int)(mismatches).size()) == 1) {
      cout << -1 << endl;
      return 0;
    }
    const int p = mismatches.front();
    mismatches.pop();
    const int q = mismatches.front();
    mismatches.pop();
    res[p] = S1[p];
    res[q] = S2[q];
    --T;
  }
  while (!mismatches.empty()) {
    const int p = mismatches.front();
    mismatches.pop();
    for (res[p] = 'a'; res[p] == S1[p] || res[p] == S2[p]; ++res[p])
      ;
    --T;
  }
  assert(0 <= T);
  for (int i = (int)(0); i < (int)(N); ++i) {
    if (res[i] != '.') {
      continue;
    }
    if (0 < T) {
      res[i] = 'a' + (S1[i] == 'a');
      --T;
    } else {
      res[i] = S1[i];
    }
  }
  cout << res << endl;
  return 0;
}