time limit per test 1 second
memory limit per test 256 megabytes
input standard input
output standard output
Even if the world is full of counterfeits, I still regard it as wonderful.
Pile up herbs and incense, and arise again from the flames and ashes of its predecessor — as is known to many, the phoenix does it like this.
The phoenix has a rather long lifespan, and reincarnates itself once every a! years. Here a! denotes the factorial of integer a, that is,a! = 1 × 2 × ... × a. Specifically, 0! = 1.
Koyomi doesn't care much about this, but before he gets into another mess with oddities, he is interested in the number of times the phoenix will reincarnate in a timespan of b! years, that is, 
. Note that when b ≥ a this value is always integer.
As the answer can be quite large, it would be enough for Koyomi just to know the last digit of the answer in decimal representation. And you're here to provide Koyomi with this knowledge.
Input
The first and only line of input contains two space-separated integers a and b (0 ≤ a ≤ b ≤ 1018).
Output
Output one line containing a single decimal digit — the last digit of the value that interests Koyomi.
Examples
input
2 4
output
2
input
0 10
output
0
input
107 109
output
2
Note
In the first example, the last digit of 
is 2;
In the second example, the last digit of 
is 0;
In the third example, the last digit of 
is 2.
【翻译】输入a,b,求b!/a! 的个位数
题解：
①直接弄会炸，但是发现只要遇到各位0就直接输出0了。
②根据上述结论，就算没有0，枚举不超过9个数。
#include<stdio.h>
using namespace std;
long long a,b,ans=1;
int main()
{
scanf("%I64d%I64d",&a,&b);
for(long long i=a+1;i<=b;i++)
{
(ans*=(i%10))%=10;
if(!ans)break;
}
printf("%I64d
",ans);return 0;
}//Paul_Guderian
也许下一回追梦时依旧会破碎，可人生绝不该任困苦摆布并凋零。
就算还有一丝呼吸我也要拼命追寻，只愿灿烂这平凡而不羁的生命。————汪峰《不羁的生命》