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();
}1395A - Boboniu Likes to Color Balls | 1637C - Andrew and Stones |
1334B - Middle Class | 260C - Balls and Boxes |
1554A - Cherry | 11B - Jumping Jack |
716A - Crazy Computer | 644A - Parliament of Berland |
1657C - Bracket Sequence Deletion | 1657B - XY Sequence |
1009A - Game Shopping | 1657A - Integer Moves |
230B - T-primes | 630A - Again Twenty Five |
1234D - Distinct Characters Queries | 1183A - Nearest Interesting Number |
1009E - Intercity Travelling | 1637B - MEX and Array |
224A - Parallelepiped | 964A - Splits |
1615A - Closing The Gap | 4C - Registration System |
1321A - Contest for Robots | 1451A - Subtract or Divide |
1B - Spreadsheet | 1177A - Digits Sequence (Easy Edition) |
1579A - Casimir's String Solitaire | 287B - Pipeline |
510A - Fox And Snake | 1520B - Ordinary Numbers |