#include<bits/stdc++.h>
#define FOR(x,a,b) for(int x=a;x<=b;x++)
#define FOD(x,a,b) for(int x=a;x>=b;x--)
#define pub push_back
#define pob pop_back
#define fi first
#define se second
#define sz(x) x.size()
#define cout(x) cout<<fixed<<setprecision(x)
#define file freopen("h.inp","r",stdin);freopen("h.out","w",stdout);
#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
typedef unsigned long long ull;
typedef long long ll;
const int N=1e6+5;
const int mod=2021;
using namespace std;
ll n,a[N];
void solve(){
    map<int,int>mp;
    ll h=0;
    FOR(i,1,n){cin>>a[i];mp[a[i]]++;}
    FOR(i,1,n)if(a[i]>0&&mp[a[i]*-1]){
        h+=mp[a[i]*-1]*mp[a[i]];
        mp[a[i]]=0;
        mp[-a[i]]=0;
    }
    h+=mp[0]*(mp[0]-1)/2;
    cout<<h<<endl;
}
int main(){
    fast
    //file
    cin>>n;
    solve();
    return 0;
}