#include <bits/stdc++.h>
using namespace std;
void FastIO() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
  cout.precision(15);
}
long long modpow(long long a, long long p, long long mod) {
  long long ret = 1;
  while (p) {
    if (p & 1) ret = (ret * a) % mod;
    a = (a * a) % mod;
    p /= 2;
  }
  return ret % mod;
}
long long power(long long a, long long p) {
  long long ret = 1;
  while (p) {
    if (p & 1) ret = (ret * a);
    a = (a * a);
    p /= 2;
  }
  return ret;
}
int main() {
  FastIO();
  int n, t;
  cin >> n >> t;
  string a, b;
  cin >> a >> b;
  int cnt = 0;
  for (int i = 0; i < n; i++)
    if (a[i] != b[i]) cnt++;
  string c;
  c.resize(n);
  if (cnt < t) {
    int req = t - cnt;
    for (int i = 0; i < n; i++) {
      if (a[i] == b[i]) {
        if (!req) {
          c[i] = a[i];
          continue;
        }
        req--;
        for (int j = 0; j < 26; j++)
          if (j != (a[i] - 'a') && j != (b[i] - 'a')) {
            c[i] = (char)(j + 'a');
            break;
          }
      } else {
        for (int j = 0; j < 26; j++)
          if (j != (a[i] - 'a') && j != (b[i] - 'a')) {
            c[i] = (char)(j + 'a');
            break;
          }
      }
    }
    cout << c << '\n';
    return 0;
  }
  int tmp = cnt - t, pos;
  if (t < (cnt + 1) / 2) {
    cout << -1 << '\n';
    return 0;
  }
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i]) {
      c[i] = a[i];
      continue;
    }
    if (!tmp) {
      pos = i;
      break;
    }
    c[i] = a[i];
    tmp--;
  }
  tmp = cnt - t;
  for (int i = pos; i < n; i++) {
    if (a[i] == b[i]) {
      c[i] = a[i];
      continue;
    }
    if (!tmp) {
      continue;
    }
    c[i] = b[i];
    tmp--;
  }
  for (int i = 0; i < n; i++) {
    if (c[i] >= 'a' && c[i] <= 'z') continue;
    for (int j = 0; j < 26; j++)
      if (j != (a[i] - 'a') && j != (b[i] - 'a')) {
        c[i] = (char)(j + 'a');
        break;
      }
  }
  cout << c << '\n';
  return 0;
}
