#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx")
using namespace std;
const long long llmx = 9223372036854775807;
const int inf = 1e9;
struct kek {
  string x;
  string y;
  string z;
};
bool isPrime(long long x) {
  if (x == 1) {
    return false;
  }
  for (long long i = 2; i <= (long long)sqrt(x); i++) {
    if (x % i == 0) {
      return false;
    }
  }
  return true;
}
long long gcd(long long a, long long b) {
  if (b == 0) {
    return a;
  }
  return gcd(b, a % b);
}
long long factorial(long long x) {
  long long sum = 1;
  for (long long i = 1; i <= x; i++) {
    sum *= i;
  }
  return sum;
}
int toint(string s) {
  int x = 0;
  for (int i = 0; i < (int)s.length(); i++) {
    x *= 10;
    x += (s[i] - '0');
  }
  return x;
}
string tostr(int x) {
  if (x == 0) {
    return "0";
  }
  string s = "";
  bool f = x < 0;
  x = abs(x);
  while (x != 0) {
    s += (x % 10) + '0';
    x /= 10;
  }
  string t = "";
  if (f) {
    t = "-";
  }
  for (int i = s.length() - 1; i >= 0; i--) {
    t += s[i];
  }
  return t;
}
long long mod = 1e9 + 9;
long long bpow(long long a, long long b) {
  if (b == 0) {
    return 1;
  } else {
    if (b % 2 == 1) {
      return bpow(a, b - 1) * a;
    } else {
      long long r = bpow(a, b / 2);
      return r * r;
    }
  }
}
string tobin(int x) {
  string s;
  while (x != 0) {
    s += (x % 2) + '0';
    x /= 2;
  }
  reverse(s.begin(), s.end());
  return s;
}
int main() {
  int n;
  cin >> n;
  map<int, long long> c;
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    c[x]++;
  }
  long long ans = 0;
  for (auto j : c) {
    if (j.first == 0) {
      ans += c[j.first] * (c[j.first] - 1) / 2;
    } else {
      if (j.first > 0) ans += c[j.first] * c[-j.first];
    }
  }
  cout << ans;
  return 0;
}
