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;
bool rev(long long x, long long y) { return x > y; }
void input_arr(long long a[], long long n) {
  for (long long i = 0; i < n; i += 1) cin >> a[i];
}
bool sortbysec(const pair<long long, long long> &a,
               const pair<long long, long long> &b) {
  return a.second < b.second;
}
bool sortinrev(const pair<long long, long long> &a,
               const pair<long long, long long> &b) {
  return (a.first > b.first);
}
void print_vec(vector<long long> &v) {
  for (auto i : v) cout << i << " ";
  cout << '\n';
}
bool sortinrevbysec(const pair<long long, long long> &a,
                    const pair<long long, long long> &b) {
  return (a.second > b.second);
}
void print_arr(long long a[], long long n) {
  for (long long i = 0; i < n; i += 1) cout << a[i] << " ";
  cout << '\n';
}
template <typename... T>
void input(T &...args) {
  ((cin >> args), ...);
}
template <typename... T>
void print(T &&...args) {
  ((cout << args << '\n'), ...);
}
template <typename... T>
void deb(T &&...args) {
  cout << "~~ ";
  ((cout << args << " "), ...);
  cout << '\n';
}
void stand8th() {
  long long n, t;
  input(n, t);
  string a, b;
  input(a, b);
  string c;
  for (long long i = 0; i < n; i += 1) c += '#';
  long long cnt = 0;
  t = n - t;
  vector<long long> v, v1;
  for (long long i = 0; i < n; i += 1) {
    if (a[i] == b[i]) {
      cnt++;
      v.push_back(i);
    } else {
      v1.push_back(i);
    }
  }
  long long T = t;
  if (t >= cnt) {
    for (auto vbn : v) c[vbn] = a[vbn];
    t -= cnt;
    long long j = 0;
    for (long long i = 0; i < t; i += 1) {
      if (j >= (long long)(v1.size())) break;
      c[v1[j]] = a[v1[j]];
      j++;
    }
    for (long long i = 0; i < t; i += 1) {
      if (j >= (long long)(v1.size())) break;
      c[v1[j]] = b[v1[j]];
      j++;
    }
    char ch = 'a';
    for (long long i = 0; i < n; i += 1) {
      if (c[i] == '#') {
        while (ch == a[i] || ch == b[i]) {
          if (ch == 'z')
            ch = 'a';
          else
            ch++;
        }
        c[i] = ch;
      }
    }
    long long cnt1 = 0, cnt2 = 0;
    for (long long i = 0; i < n; i += 1) {
      if (a[i] == c[i]) cnt1++;
      if (b[i] == c[i]) cnt2++;
    }
    if (cnt1 != cnt2 || cnt1 != T) {
      print(-1);
      return;
    }
    print(c);
    return;
  }
  for (long long i = 0; i < t; i += 1) {
    c[v[i]] = a[v[i]];
  }
  char ch = 'a';
  for (long long i = 0; i < n; i += 1) {
    if (c[i] == '#') {
      while (ch == a[i] || ch == b[i]) {
        if (ch == 'z')
          ch = 'a';
        else
          ch++;
      }
      c[i] = ch;
    }
  }
  long long cnt1 = 0, cnt2 = 0;
  for (long long i = 0; i < n; i += 1) {
    if (a[i] == c[i]) cnt1++;
    if (b[i] == c[i]) cnt2++;
  }
  if (cnt1 != cnt2 || cnt1 != T) {
    print(-1);
    return;
  }
  print(c);
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long tin = 1;
  while (tin--) {
    stand8th();
  }
  return 0;
}