O(logn)算法

#include<cstdio>
long long ans;

const int p = 5;
const int sp=(p*(p-1))>>1;
int main() {
    int n;
    scanf("%d",&n);
    int s=n,l=0,base1=p,base=1;
    while(s/=5) l++;
    for(register int cur=1;cur<=l;cur++) {
        int base2=base1*p,x=(n+1)/base2;
        long long a=sp*x*base1;
        int y=n%base2-base1+1;
        for(register int i=1;i<p&&y>0;i++,y-=base1) {
            if(y>base1) a+=base1*i;
            else { a+=y*i; break; }
        }
        base1=base2;
        ans+=a*base; base=p*base+1;
    }
    printf("%lld\n",ans);
    return 0;
}