#include <bits/stdc++.h>
using namespace std;
int readInt() {
  bool minus = false;
  int result = 0;
  char ch;
  ch = getchar();
  while (true) {
    if (ch == '-') break;
    if (ch >= '0' && ch <= '9') break;
    ch = getchar();
  }
  if (ch == '-')
    minus = true;
  else
    result = ch - '0';
  while (true) {
    ch = getchar();
    if (ch < '0' || ch > '9') break;
    result = result * 10 + (ch - '0');
  }
  if (minus)
    return -result;
  else
    return result;
}
template <class T>
T mulmod(T a, T b) {
  if (b == 0) return 0;
  if (b == 1) return a;
  T x = mulmod(a, b >> 1);
  x = (x + x) % 1000000007;
  if (b & 1) x = (x + a) % 1000000007;
  return x;
}
template <class T>
T larger(T a, T b) {
  return (a > b ? a : b);
}
template <class T>
T smaller(T a, T b) {
  return (a < b ? a : b);
}
template <class T>
T gcd(T a, T b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
template <class T>
T lcm(T a, T b) {
  return (a * b) / gcd(a, b);
}
template <class T>
string converter(T n) {
  stringstream x;
  x << n;
  return x.str();
}
template <class T>
T pw(T a, T b) {
  if (b == 0) return 1;
  T x = pw(a, b >> 1);
  x = (x * x) % 1000000007;
  if (b & 1) x = (x * a) % 1000000007;
  return x;
}
template <class T1>
void deb(T1 e1) {
  cout << e1 << endl;
}
template <class T1, class T2>
void deb(T1 e1, T2 e2) {
  cout << e1 << " " << e2 << endl;
}
template <class T1, class T2, class T3>
void deb(T1 e1, T2 e2, T3 e3) {
  cout << e1 << " " << e2 << " " << e3 << endl;
}
template <class T1, class T2, class T3, class T4>
void deb(T1 e1, T2 e2, T3 e3, T4 e4) {
  cout << e1 << " " << e2 << " " << e3 << " " << e4 << endl;
}
template <class T1, class T2, class T3, class T4, class T5>
void deb(T1 e1, T2 e2, T3 e3, T4 e4, T5 e5) {
  cout << e1 << " " << e2 << " " << e3 << " " << e4 << " " << e5 << endl;
}
template <class T1, class T2, class T3, class T4, class T5, class T6>
void deb(T1 e1, T2 e2, T3 e3, T4 e4, T5 e5, T6 e6) {
  cout << e1 << " " << e2 << " " << e3 << " " << e4 << " " << e5 << " " << e6
       << endl;
}
char totalDiff(char a, char b) {
  for (char c = 'a'; c <= 'z'; c++)
    if (c != a && c != b) return c;
}
void letsgo() {
  int n, t;
  cin >> n >> t;
  string a, b, c;
  cin >> a >> b;
  int i, same = 0;
  for (i = 0; i < n; i++) {
    same += (a[i] == b[i]);
  }
  c = "";
  if (same >= n - t) {
    int j = n - t;
    for (i = 0; i < n; i++) {
      if (a[i] == b[i] && j)
        c.push_back(a[i]), j--;
      else
        c.push_back(totalDiff(a[i], b[i]));
    }
    cout << c;
    return;
  }
  int j = n - t - same;
  int l = j;
  int k = same;
  if (2 * j > n - same) {
    cout << -1;
    return;
  }
  for (i = 0; i < n; i++) {
    if (a[i] == b[i] && k) {
      c.push_back(a[i]);
      k--;
    } else if (j) {
      c.push_back(a[i]);
      j--;
    } else if (l) {
      c.push_back(b[i]);
      l--;
    } else {
      c.push_back(totalDiff(a[i], b[i]));
    }
  }
  cout << c;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  ;
  letsgo();
  int t;
  cin >> t;
  return 0;
}
