#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
const ll inf = 1;
const ll mod = 1E9;
int main() {
  ios::sync_with_stdio(false);
  string a, b;
  int n, k;
  cin >> n >> k;
  cin >> a >> b;
  int mtch = 0;
  k = n - k;
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i]) {
      mtch++;
    }
  }
  int mx = mtch + (n - mtch) / 2;
  if (k > mx)
    cout << -1 << endl;
  else {
    string c;
    for (int i = 0; i < n; i++) {
      for (int j = 'a'; j <= 'z'; j++) {
        if (j != a[i] && j != b[i]) {
          c += (char)j;
          break;
        }
      }
    }
    for (int i = 0; i < n && k; i++) {
      if (a[i] == b[i]) {
        c[i] = a[i];
        k--;
      }
    }
    int k1 = k, i;
    for (i = 0; i < n && k1; i++) {
      if (a[i] != b[i]) {
        c[i] = a[i];
        k1--;
      }
    }
    k1 = k;
    for (; i < n && k1; i++) {
      if (a[i] != b[i]) {
        c[i] = b[i];
        k1--;
      }
    }
    cout << c << endl;
  }
  return 0;
}
