Cocktail

#include <bits/stdc++.h>

using namespace std;

long long int tail[200020];
int main(){
    int n;
    scanf("%d",&n);
    int y;
    for(int i=0;i<n;++i){
        scanf("%d",&y);
        tail[y]++;
    }
    int a;
    scanf("%d",&a);
    long long int  ans=0;
    long long int x;
    if(a%2==0){
        x=a/2;
    }
    else{
        x=(a+1)/2;
    }
    for(int i=a;i>=x;--i){
        if(i%2==0&&i==a/2){
            ans=ans+(((tail[i]-1)*(tail[i]))/2);
        }
        else{
             ans=ans+(tail[i]*tail[a-i]);
        }

    }
    printf("%lld",ans);

    return 0;
}