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;
int Set(int N, int pos) { return N |= (1LL << pos); }
int Reset(int N, int pos) { return N &= ~(1LL << pos); }
bool Check(int N, int pos) { return N & (1LL << pos); }
int dx[] = {0, 0, 1, -1, 1, -1, 1, -1}, dy[] = {1, -1, 0, 0, 1, -1, -1, 1};
inline void cn(long &n) {
  n = 0;
  long ch = getchar();
  int sign = 1;
  while (ch < '0' || ch > '9') {
    if (ch == '-') sign = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9')
    n = (n << 3) + (n << 1) + ch - '0', ch = getchar();
  n = n * sign;
}
template <class T>
void cmin(T &a, T b) {
  if (b < a) a = b;
}
template <class T>
void cmax(T &a, T b) {
  if (b > a) a = b;
}
template <class T>
int len(const T &c) {
  return (int)c.size();
}
template <class T>
int len(char c[]) {
  return (int)strlen(c);
}
long stoi(string s) {
  long n = 0;
  for (int i(0), _n(len(s)); i < _n; ++i) n = n * 10 + (s[i] - 48);
  return n;
}
int main() {
  ios_base::sync_with_stdio(false);
  long n, t;
  string s, second, res = "";
  cin >> n >> t;
  cin >> s >> second;
  for (int i(0), _n(n); i < _n; ++i) res += ".";
  long m = n - t;
  long c = 0;
  for (int i(0), _n(n); i < _n; ++i) {
    if (s[i] == second[i]) c++;
  }
  if (n == c) {
    if (t == 0)
      cout << s;
    else {
      for (int i(0), _n(n); i < _n; ++i) {
        if (m) {
          res[i] = s[i];
          m--;
        } else {
          if (s[i] != 'a' && second[i] != 'a')
            res[i] = 'a';
          else if (s[i] != 'b' && second[i] != 'b')
            res[i] = 'b';
          else if (s[i] != 'c' && second[i] != 'c')
            res[i] = 'c';
          else
            s[i] = 'd';
        }
      }
      cout << res;
    }
  } else if (((n - c) / 2) < (m - c)) {
    cout << -1;
  } else {
    for (int i(0), _n(n); i < _n; ++i) {
      if (m == 0) break;
      if (s[i] == second[i]) {
        res[i] = second[i];
        m--;
      }
    }
    long mm = m;
    for (int i(0), _n(n); i < _n; ++i) {
      if (m == 0) break;
      if (res[i] != '.') continue;
      res[i] = s[i];
      m--;
    }
    m = mm;
    for (int i(0), _n(n); i < _n; ++i) {
      if (m == 0) break;
      if (res[i] != '.') continue;
      res[i] = second[i];
      m--;
    }
    for (int i(0), _n(n); i < _n; ++i) {
      if (res[i] != '.') continue;
      if (s[i] != 'a' && second[i] != 'a')
        res[i] = 'a';
      else if (s[i] != 'b' && second[i] != 'b')
        res[i] = 'b';
      else if (s[i] != 'c' && second[i] != 'c')
        res[i] = 'c';
      else
        s[i] = 'd';
    }
    cout << res;
  }
  return 0;
}