[Bzoj3505][Cqoi2014]数三角形

#include<iostream>#include<cstdio>using namespace std;int n,m;long long c[1001*1001+1][4],ans,tmp;int read(){    int x=0;char ch=getchar();    while(ch<'0'||ch>'9'){ch=getchar();}    while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}    return x;}inline int gcd(int a,int b){return b==0?a:gcd(b,a%b);}void calc(){c[0][0]=1;for(int i=1;i<=n*m;i++){c[i][0]=1;for(int j=1;j<=3;j++)c[i][j]=c[i-1][j]+c[i-1][j-1];}}void solve(){ans=c[n*m][3];ans-=n*c[m][3];ans-=m*c[n][3];for(int i=1;i<n;i++)for(int j=1;j<m;j++){tmp=gcd(i,j)+1;if(tmp>2)ans-=(tmp-2)*2*(n-i)*(m-j);}printf("%lld",ans);}int main(){n=read();m=read();n++;m++;calc();solve();return 0;}