#include <iostream>
using namespace std;

int main() 
{
	int B[10];


	for (int i = 1; i < 10; i++)
	{
		cin >> B[i];

		if (B[i] != i)
		{
			cout << B[i]-1 << endl;
			break;
		}

		if (B[i] == 9)
		{
			cout << B[i] + 1 << endl;
		}
	}
	return 0;
}