#include <bits/stdc++.h>
using namespace std;

int main() {
  int B[10];
  cin >> B[0] >> B[1] >> B[2] >> B[3] >> B[4] >> B[5] >> B[6] >> B[7] >> B[8];
  B[9] = 0;
  for (int i = 0; i < 10; i++) {
    if (B[i] != i + 1) {
      cout << i + 1 << endl;
      break;
    }
  }
}