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≤M≤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
这里写图片描述
As the picture is shown, when you select M1 and M2, M1 show the color of K1, and M2 show the color of K2.
When you select M3 and M2, M2 show the color of K1 and M3 show the color of K2.
When you select M1 and M3, M1 show the color of K1.
题目分析:对于每个测试样例给两个整数,分别是M个display screens,K个different signal sources,(K≤M≤2^32−1),从M个screens中选出K个screens,在源头和屏幕之间连线,使得这K个屏幕可以显示出不同的K中颜色。
连线只存在源头和屏幕之间。