#include <bits/stdc++.h>
using namespace std;
struct debugger {
  template <typename T>
  debugger& operator,(const T& v) {
    cerr << v << " ";
    return *this;
  }
} dbg;
long long mod = 1000000007;
long long mod_v(long long nn) { return nn % mod; }
template <class T>
T fast_pow(T n, T p) {
  if (p == 0) return 1;
  if (p % 2) {
    T g = mod_v(mod_v(n) * mod_v(fast_pow(n, p - 1)));
    return g;
  } else {
    T g = fast_pow(n, p / 2);
    g = mod_v(mod_v(g) * mod_v(g));
    return g;
  }
}
template <class T>
inline T modInverse(T n) {
  return fast_pow(n, mod - 2);
}
char a[1111111], b[1111111], c[1111111];
int n, t;
bool chk[1111111];
int main() {
  scanf("%d %d ", &n, &t);
  scanf("%s", &a);
  scanf("%s", &b);
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < 26; j++) {
      char p = (char)(j + 'a');
      if (a[i] != p && b[i] != p) {
        c[i] = p;
        break;
      }
    }
  }
  t = n - t;
  for (int i = 0; i < n && t; i++) {
    if (a[i] == b[i]) {
      if (!chk[i]) {
        c[i] = a[i];
        t--;
        chk[i] = 1;
      }
    }
  }
  if (t) {
    int ta = t;
    int tb = t;
    int turn = 0;
    for (int i = 0; i < n && (ta || tb); i++) {
      if (chk[i]) continue;
      if (turn) {
        c[i] = a[i];
        turn = !turn;
        ta--;
      } else {
        c[i] = b[i];
        turn = !turn;
        tb--;
      }
    }
    t = max(tb, ta);
  }
  if (t) {
    cout << "-1\n";
    return 0;
  }
  for (int i = 0; i < n; i++) {
    cout << c[i];
  }
  cout << endl;
  return 0;
}
