#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 5;
int n, v[N];
void Print() {
  for (int i = 1; i <= n; i++) cout << char(v[i] + 'a');
  exit(0);
}
int main() {
  int need;
  cin >> n >> need;
  string a, b;
  cin >> a >> b;
  a = "x" + a;
  b = "x" + b;
  for (int i = 1; i <= n; i++) v[i] = -1;
  need = n - need;
  for (int i = 1; i <= n; i++) {
    if (need == 0) break;
    if (a[i] == b[i]) {
      v[i] = a[i] - 'a';
      need--;
    }
  }
  if (need == 0) {
    for (int i = 1; i <= n; i++) {
      if (v[i] != -1) continue;
      v[i] = 0;
      while (v[i] == a[i] - 'a' || v[i] == b[i] - 'a') v[i]++;
    }
    Print();
  }
  int have = 0;
  for (int i = 1; i <= n; i++)
    if (v[i] == -1) have++;
  if (2 * need > have) {
    cout << "-1\n";
    return 0;
  }
  int F = need, S = need;
  for (int i = 1; i <= n; i++) {
    if (v[i] != -1) continue;
    if (F) {
      v[i] = a[i] - 'a';
      F--;
      continue;
    }
    if (S == 0) break;
    v[i] = b[i] - 'a';
    S--;
  }
  for (int i = 1; i <= n; i++) {
    if (v[i] != -1) continue;
    v[i] = 0;
    while (v[i] == a[i] - 'a' || v[i] == b[i] - 'a') v[i]++;
  }
  Print();
  return 0;
}
