Uri 2129 - Factorial

#include<bits/stdc++.h>

using namespace std;
//last non zero digit any factorial number function
long long fac(int a){
    int f[11] = {1,1,2,6,4,2,2,4,2,8,8};
        // mathematical equation
       // a/10 == odd  fac(a) = 4*fac(a/5)*fac(a%10)
      // a/10 == even  fac(a) = 6*fac(a/5)*fac(a%10)
    if(a < 11) return f[a];
    return ( ( a / 10 ) % 2 == 1?  4 : 6 ) * fac( a / 5 ) * fac ( a % 10 );      
}

int main()
{
    int n , i = 1;
    while(scanf("%d",&n) != EOF){
    printf("Instancia %d\n%d\n\n",i++,( fac(n) ) % 10);
    }
    return 0;
}