#include<stdio.h>
int main()
{
    int n,sum=10,i;
    for(i=2;i<=100;i++){
        sum=sum*10;
    }
    char buf[sum];
    scanf("%d",&n);
    sprintf(buf,"%d",n);
    i=0;
    while(1){
        if(buf[i]=='\0'){
            if(buf[i-1]=='3'){
                printf("%d\n",i);
                break;
            }else{
                printf("-1\n");
                break;
            }
        }
        if(buf[i]!='3'){
            printf("-1\n");
            break;
        }
        i++;
    }
    return 0;
}