#include<bits/stdc++.h>
#define LL long long
using namespace std;
unordered_map<LL, LL>mymap;
LL arr[10002];
int main()
{
int n;
scanf("%d", &n);
for(int i=0; i<n; i++){
scanf("%lld", arr+i);
mymap[2*arr[i]]++;
}
LL cnt = 0;
for(int i=0; i<n; i++){
for(int j=i+1; j<n; j++){
LL val = mymap[arr[i] + arr[j]];
if(val>0) cnt = cnt + val;
mymap[arr[i] + arr[j]] = 0;
}
}
printf("%lld", cnt);
}
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