n = input()
pari = {}
paruDZ = 0
saraksts = list(map(int,input().split()))
for i in saraksts:
if i in pari:
pari[i]+=1
else:
pari[i] = 1
for k in pari:
if k == 0 :
paruDZ += pari[k] * (pari[k] - 1)
elif k * -1 in pari:
paruDZ += pari[k] * pari[k * -1]
print(paruDZ //2)
#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <string.h>
#include<bitset>
#include<cmath>
#include<algorithm>
#include<iomanip>
#include<vector>
#include<stack>
#include<queue>
#include<deque>
#include<set>
#include<map>
#include<string>
#include<cstdlib>
#define endl '\n'
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define test_case int t;cin>>t;while(t--)
typedef long long int ll;
typedef long double ld;
using namespace std;
void Abdo_Saad() { std::ios_base::sync_with_stdio(0);cin.tie(NULL);cout.tie(NULL); }
ll gcd(ll a, ll b) { return(b == 0 ? a : gcd(b, a % b)); }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
void set_bit(int& n, int idx) { n |= 1 << idx; }
void reset_bit(int& n, int idx) { n &= ~(1 << idx); }
void flib_bit(int& n, int idx) { n ^= 1 << idx; }
void solve()
{
int n;
cin >> n;
vector<int>v(n),freq(11);
for (int i = 0;i < n;i++)
{
cin >> v[i];
if(v[i]>=0)freq[v[i]]++;
}
ll cnt = 0;
for (int i = 0;i < n;i++)
{
if (v[i] < 0)cnt += freq[v[i] * -1];
}
cout << cnt + ll(freq[0]) * (freq[0] - 1) / 2;
}
int main()
{
Abdo_Saad();
//freopen("strings.in", "r", stdin);
//freopen(".out","w",stdout);
//test_case
solve();
}620A - Professor GukiZ's Robot | 1342A - Road To Zero |
1520A - Do Not Be Distracted | 352A - Jeff and Digits |
1327A - Sum of Odd Integers | 1276A - As Simple as One and Two |
812C - Sagheer and Nubian Market | 272A - Dima and Friends |
1352C - K-th Not Divisible by n | 545C - Woodcutters |
1528B - Kavi on Pairing Duty | 339B - Xenia and Ringroad |
189A - Cut Ribbon | 1182A - Filling Shapes |
82A - Double Cola | 45A - Codecraft III |
1242A - Tile Painting | 1663E - Are You Safe |
1663D - Is it rated - 3 | 1311A - Add Odd or Subtract Even |
977F - Consecutive Subsequence | 939A - Love Triangle |
755A - PolandBall and Hypothesis | 760B - Frodo and pillows |
1006A - Adjacent Replacements | 1195C - Basketball Exercise |
1206A - Choose Two Numbers | 1438B - Valerii Against Everyone |
822A - I'm bored with life | 9A - Die Roll |