2659: [Beijing wc2012]算不出的算式 打表+找规律

打表找一下规律。如果$p=q$则返回$\frac{p+1}{2}\times\frac{q}{2}$否则返回$\frac{p}{2}\times\frac{q}{2}$

#include<iostream>
#include<cstdio>
using namespace std;
int prime[21];
bool flag[21];
const int N=20;
long long p,q;
inline int read()
{
    int a=0,f=1; char c=getchar();
    while (c<'0'||c>'9') {if (c=='-') f=-1; c=getchar();}
    while (c>='0'&&c<='9') {a=a*10+c-'0'; c=getchar();}
    return a*f;
}
void pre()
{
    for (int i=2;i<=N;i++)
    {
        if (!flag[i]) prime[++prime[0]]=i;
        for (int j=1;j<=prime[0]&&i*prime[j]<=N;j++)
        {
            flag[i*prime[j]]=1;
            if (i%prime[j]==0) break;
        }
    }
}
inline int calc(int p,int q)
{
    int sum=0;
    for (int k=1;k<=p-1>>1;k++)
        sum+=k*q/p;
    for (int l=1;l<=q-1>>1;l++)
        sum+=l*p/q;
    return sum;
}
int main()
{
/*  pre();  
    for (int i=2;i<=prime[0];i++)
        for (int j=2;j<=prime[0];j++)
            printf("%d %d %d\n",prime[i],prime[j],calc(prime[i],prime[j]));*/
    scanf("%lld%lld",&p,&q);
    if (p==q) p++;
    cout << (p>>1)*(q>>1);
    return 0;
}