bzoj 2005

    裸的2D gcd。ans=(Σ(d<=n)phi[d]*(n/d)*(m/d))*2-n*m;

    

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#define int long long
using namespace std;
int n,m;
int pr[100005],su[100005],cnt,phi[100005];
void shai()
{
    phi[1]=1;
    for(int i=2;i<=100000;i++)
    {
        if(!phi[i])su[++cnt]=i,pr[i]=i,phi[i]=i-1;
        for(int j=1;su[j]<=pr[i]&&su[j]*i<=100000&&j<=cnt;j++)
        {
              pr[su[j]*i]=su[j];
              if(su[j]==pr[i])phi[su[j]*i]=phi[i]*su[j];
              else phi[su[j]*i]=phi[i]*(su[j]-1);
        }
    }
}
signed main()
{
   shai();
   scanf("%lld%lld",&n,&m);int ans=0;
   for(int i=1;i<=min(n,m);i++)ans+=phi[i]*(n/i)*(m/i);
   ans*=2;ans-=(n*m);
   printf("%lld
",ans);
   return 0;
}