最快

#include <regex>
int a[10001],x,num,n;int main(){scanf("%d",&n);for(int i=1;i<=n;i++){scanf("%d",&x);a[x];}for(int i=1;i<=10000;i){if(a[i]==0)continue;for(int j=1;j<i-j;j++)if(a[j]==1&&a[i-j]==1){num++;break;}}printf("%d",num);}