思路：先求出所有的逆序对数目。如果小于k，说明所有的逆序对都可以被交换消去，输出0；否则只能消去k个，输出 逆序对数-k。求逆序对用归并排序。

#include <iostream>

using namespace std;

const int N = 100010;

typedef long long LL;

int n, k;
int q[N], tmp[N];

LL merge_sort(int q[], int l, int r)
{
    if (l >= r) return 0;
    int mid = l + r >> 1;
    LL res = merge_sort(q, l, mid) + merge_sort(q, mid + 1, r);
    int i = l, j = mid + 1, k = 0;
    while (i <= mid && j <= r)
    {
        if (q[i] <= q[j]) tmp[k ++] = q[i ++];
        else
        {
            tmp[k ++] = q[j ++];
            res += mid - i + 1;
        }
    }
    while (i <= mid) tmp[k ++] = q[i ++];
    while (j <= r) tmp[k ++] = q[j ++];
    for (int i = l, j = 0; i <= r; i ++, j ++) q[i] = tmp[j];
    return res;
}

int main()
{
    while (cin >> n >> k)
    {
        for (int i = 0; i < n; i ++) cin >> q[i];
        LL cnt = merge_sort(q, 0, n - 1);
        if (cnt <= k) cout << "0" << endl;
        else cout << cnt - k << endl;
    }
    return 0;
}