SGU 222 Little Rooks （组合数）

222. Little Rooks

time limit per test: 0.25 sec.
memory limit per test: 65536 KB

input: standard
output: standard

Inspired by a "Little Bishops" problem, Petya now wants to solve problem for rooks. 

A rook is a piece used in the game of chess which is played on a board of square grids. A rook can only move horizontally and vertically from its current position and two rooks attack each other if one is on the path of the other. 

Given two numbers n and k, your job is to determine the number of ways one can put k rooks on an n × n chessboard so that no two of them are in attacking positions. 

Input

The input file contains two integers n (1 ≤ n ≤ 10) and k (0 ≤ k ≤ n ²). 

Output

Print a line containing the total number of ways one can put the given number of rooks on a chessboard of the given size so that no two of them are in attacking positions. 

Sample test(s)

Input

4 4 

Output

24 

#include <iostream>
#include <cstdio>
using namespace std;

int n, k;
long long ans;

int main() {
    while (~scanf("%d%d", &n, &k)) {
        if (k > n) {
            printf("%d\n", 0);
            continue;
        }
        ans = 1;
        for (int i = n; i >= n-k+1; i--) {
            ans *= i;
        }
        ans = ans * ans;
        for (int i = k; i > 1; i--) {
            ans /= i;
        }
        printf("%lld\n", ans);
    }
    return 0;
}