#include<stdio.h>
int main(void) {

	int array[9];
	int j;
	for (j = 0;j < 9;j++) {
		scanf("%d", &array[j]);
	}
	

		if (array[j + 1] - array[j] == 2)
			printf("%d\n", array[j] + 1);
	else if (array[0] != 1)
		printf("%d\n", 1);

	else  if (array[9] != 10)
		printf("%d\n", 10);
		return 0;
}