Codeforces 395A: Skis. 2-Sum problem. 1 solution for both 395A1 and 395A2. (http://codeforces.com/problemset/problem/395/A1, http://codeforces.com/problemset/problem/395/A2)

395A.cpp

// 395A1 & 395A2 - Skis
// http://codeforces.com/problemset/problem/395/A1
// http://codeforces.com/problemset/problem/395/A2

#include <cstdlib>
#include <iostream>
#include <algorithm>

const int N = 1111111;

int n, q, arr[N];

using namespace std;

int main()
{
    cin >> n >> q;

    for (int i = 0; i < n; ++i)
        cin >> arr[i];

    sort(arr, arr + n);

    int ans = 0;

    {
        int i = 0, j = n - 1;

        while (i < j)
        {
            if (arr[i] + arr[j] > q)
                --j;
            else if (arr[i] + arr[j] < q)
                ++i;
            else
                --j, ++i, ++ans;
        }
    }

    cout << ans << endl;

    return EXIT_SUCCESS;
}