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#include <bits/stdc++.h>
using namespace std;
 
unordered_map<int,int> cnt1,cnt2;
 
long long n,ans;
int a[1005];
 
int main()
{
    cin>>n;
    for(int i=0;i<n;i++)
    {
        cin>>a[i];
        cnt1[a[i]]++;
    }
    for(int i=0;i<n;i++)
        for(int j=i+1;j<n;j++)
            cnt2[a[i]+a[j]]++;
    for(int i=0;i<n;i++)
        for(int j=i+1;j<n;j++)
        {
            if(a[i]!=a[j])
                ans += cnt2[a[i]+a[j]]-cnt1[a[i]]-cnt1[a[j]]+1;
            else
                ans += cnt2[a[i]+a[j]]-cnt1[a[i]]-cnt1[a[j]]+3;
        }
    cout<<ans<<endl;
    return 0;
}
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