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[],su[],cnt,phi[];
 void shai()
 {
     phi[]=;
     for(int i=;i<=;i++)
     {
         if(!phi[i])su[++cnt]=i,pr[i]=i,phi[i]=i-;
         for(int j=;su[j]<=pr[i]&&su[j]*i<=&&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]-);
         }
     }
 }
 signed main()
 {
    shai();
    scanf("%lld%lld",&n,&m);int ans=;
    for(int i=;i<=min(n,m);i++)ans+=phi[i]*(n/i)*(m/i);
    ans*=;ans-=(n*m);
    printf("%lld\n",ans);
    return ;
 }