HDU

cable cable cable

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≤232−1). 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

3 220 15

 

Sample Output

490
Hint
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.
 

 

Source

2017 ACM/ICPC Asia Regional Shenyang Online

 

解题思路：读题

#include<iostream>#include<deque>#include<memory.h>#include<stdio.h>#include<map>#include<string>#include<algorithm>#include<vector>#include<math.h>#include<stack>#include<queue>#include<set>#define INF 1<<29using namespace std;typedef long long int ll;int main (){    ll m,k;    while(~scanf("%I64d%I64d",&m,&k))        printf("%I64d\n",k*(m-k+1));    return 0;}