#include<stdio.h>

int main(void){

int i,j,c[9];

for(i=0;i<9;i++){
	scanf("%d",&c[i]);
}


for(i=0;i<9;i++){
	if(i+1 != c[i]){
		printf("%d",i+1);
		break;
	}
}

return 0;
}