int main(){
long a,b,p,q,t;
scanf("%ld%ld",&a,&b);
if(a%b<1){for(a/=b;a%10;a/=10);printf("%d",a%10);return 0;}
for(p=a,q=b;q;q=t)t=p%q,p=q;
a/=p;b/=p;
if(b%2<1)puts("5");else{for(a%=10;b&b-1;a*=2)b/=5;printf("%d",b?a%10:-1);}
return 0;
}