CCF 相反数

import java.util.Scanner;

public class Main {
public static void main(String[] args) {
new Main().run();
}

public void run(){
Scanner sc = new Scanner(System.in);
int N = sc.nextInt();

int[] arr =new int
;
for(int i=0;i<N;i++){
arr[i]=sc.nextInt();
}

int count=0;
for(int i=0;i<N;i++){
for(int j=N-1;j>=0;j--){
if(arr[i]+arr[j]==0){
count++;
}
}
}System.out.println(count/2);
}
}