BZOJ 1800 [Ahoi2009]fly 飞行棋

//Twenty
#include<cstdio>
#include<cstdlib>
#include<iostream>
#include<algorithm>
#include<cmath>
#include<cstring>
#include<queue>
#include<vector>
typedef long long LL;
using namespace std;
int n,a[25],sum,now;
int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++) {
        scanf("%d",&a[i]);
        sum+=a[i];
    }
    if(sum&1) printf("0\n");
    else {
        sum>>=1;
        int j=1,ans=0;
        for(int i=1;i<n;i++) {
            now+=a[i];
            while(now>sum) {
                now-=a[j];
                j++;
            }
            if(now==sum) ans++;
        }
        printf("%d\n",ans*(ans-1)/2);
    }
    return 0;
}