PROG-T1090: Cock Tail

#include <bits/stdc++.h>
using namespace std;

typedef long long lli;

const int T = 1e5;

int chicken[T + 10];

int main(){

    int nChicken;
    int mx = 0;
    scanf("%d", &nChicken);
    for(int i = 1; i <= nChicken; ++i){
        int x;
        scanf("%d", &x);
        ++chicken[x];
        mx = max(mx, x);
    }

    int tr;
    scanf("%d", &tr);
    lli cnt = 0;
    for(int i = 0; i <= tr >> 1; ++i){
        if(tr - i > mx){
            continue;
        }
        if(i + i == tr){
            cnt += (lli)chicken[i] * (chicken[i] - 1) / 2;
        } else {
            cnt += (lli)chicken[i] * chicken[tr - i];
        }
    }
    cout << cnt;

    return 0;
}