HDU - cable cable cable(數學)

題目連結:http://acm.hdu.edu.cn/showproblem.php?pid=6195

Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 32768/32768 K (Java/Others)

Problem Description

Connecting the display screen and signal sources which produce different color signals by cables, then the display screen can show the color of the signal source.Notice that every signal source can only send signals to one display screen each time. 

Now you have M display screens and K different signal sources(K \leq M \leq 2^{32}-1). Select K display screens from M display screens, how many cables are needed at least so that **any** K display screens you select can show exactly K different colors.

Input

Multiple cases (no more than 100), for each test case:

there is one line contains two integers M and K.

Output

Output the minimum number of cables N.

Sample Input

3 2

20 15

Sample Output

4

90

Hint

Problem solving report:

/* 
 * @Author: lzyws739307453 
 * @Language: C++ 
 */
#include <bits/stdc++.h>
using namespace std;
int main() {
    long long m, k;
    while (~scanf("%lld%lld", &m, &k))
        printf("%lld\n", (m - (k - 1)) * k);
    return 0;
}