#include <algorithm>
#include<iostream>
#include <cstdio>
#include <ctime>
#include <vector>
#include <complex>
using namespace std;
#define cd complex<double>
#define vcd vector<cd>
#define si(n) scanf("%d",&n)
#define f(i,a,b) for(int i=a;i<b;i++)
#define pb push_back
#define PI 3.1415926535897932384626433832795
#define mm 30001
#define LL long long
int prev[mm],next[mm];
int main()
{
int n;
si(n);
int A[n];
f(i,0,n)
{
si(A[i]);
next[A[i]]++;
}
LL cnt=0;
f(j,0,n)
{
next[A[j]]--;
for(int i=max(1,2*A[j]-30000);i<=min(30000,2*A[j]-1);i++)
{
cnt+=(LL)prev[i]*next[2*A[j]-i];
}
prev[A[j]]++;
}
printf("%lld\n",cnt);
return 0;
}
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