long a,b,p,q,t;main(){scanf("%ld%ld",&a,&b);for(p=a,q=b;q;q=t)t=p%q,p=q;for(a/=p,b/=p;a%10<1;a/=10);t=b%2*3+2;for(a%=10;b%t<1;a*=2)b/=t;a=!printf("%d",b-1?-1:t-2?a%10:5);}