#include<stdio.h>
int main(void){
	int i, b[10], count;
	for(i = 0 ; i < 9 ; i++){
		scanf("%d", &b[i]);
	}
	for(count = 0 ; count < 10 ; count++){
		if(b[count] != (count+1)){
			printf("%d", count+1);
			break;
		}
	}
	return 0;
}