C++ :

#include<bits/stdc++.h>
using namespace std;

#define LL long long

const int Maxn = 5005,Maxv = 2e7+5;

int A[Maxn] , cnt[Maxv] , C[Maxv];
int N;

inline LL C2(int x){return (LL)x*(x-1) / 2;}
inline LL C3(int x){return (LL)x*(x-1)*(x-2)/6;}
inline LL C4(int x){return (LL)x*(x-1)*(x-2)*(x-3)/24;}

int main(){
	scanf("%d",&N);
	for(int i = 1 ; i <= N ; ++i)
		scanf("%d",&A[i]),cnt[A[i]]++;
	sort(A + 1 , A + N + 1);
	LL ans = 0;

	for(int i = 1 ; i <= N ; ++i){
		LL num = cnt[A[i]];
		if(A[i] != A[i-1]){
			for(int j = i + 1 ; j <= N && A[j] < 2*A[i] ; ++j) if(A[j] != A[j+1])
				ans += C2(cnt[A[j]]) * (num * cnt[A[j] - A[i]] * C[A[j]] + C2(num) * C2(cnt[A[j]-A[i]]));
			ans += C2(cnt[A[i] << 1]) * C4(cnt[A[i]]);
		}
		for(int j = i + 1 ; j <= N ; ++j) if(A[j+1] != A[j]) 
			ans += C3(cnt[A[j]])*C[A[j] - A[i]];
		for(int j = 1 ; j < i ; ++j)
			++C[A[j] + A[i]];
	}
	printf("%lld\n",ans);
	return 0;
}