Submission #8800108

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Source Code Expand

#include<cstdio>
using namespace std;
typedef long long LL;
const int maxn=200005,TT=1000000007;
int n,F[4005][4005],fac[8005],inv[8005],A[maxn],B[maxn],ans;
inline int read()
{
	int ret=0,f=1;char ch=getchar();
	while(ch<'0'||ch>'9'){if(ch=='-')f=-f;ch=getchar();}
	while(ch>='0'&&ch<='9'){ret=ret*10+ch-'0';ch=getchar();}
	return ret*f;
}
inline int QP(int a,int b)
{
	int ret=1,w=a;
	while(b)
	{
		if(b&1) ret=(LL)ret*w%TT;
		w=(LL)w*w%TT;b>>=1;
	}
	return ret;
}
inline int C(int n,int m){return (LL)fac[n]*inv[m]%TT*inv[n-m]%TT;}
int main()
{
	n=read();fac[0]=1;
	for(int i=1;i<=8000;i++){fac[i]=(LL)fac[i-1]*i%TT;inv[i]=QP(fac[i],TT-2);}
	for(int i=1;i<=n;i++)
	{
		A[i]=read();B[i]=read();
		F[2000-A[i]][2000-B[i]]++;
	}
	for(int i=0;i<=4000;i++)
		for(int j=0;j<=4000;j++)
			F[i][j]=((LL)F[i][j]+(j?F[i][j-1]:0)+(i?F[i-1][j]:0))%TT;
	for(int i=1;i<=n;i++)
	{
		ans=(ans+F[2000+A[i]][2000+B[i]])%TT;
		ans=(ans-C(A[i]+B[i]+A[i]+B[i],A[i]+A[i])+TT)%TT;
	}
	ans=(LL)ans*QP(2,TT-2)%TT;
	printf("%d\n",ans);
	return 0;
}

Submission Info

Judge Result