Problem:
Score : 100 points

Problem Statement

Determine if we can choose K different integers between 1 and N (inclusive) so that no two of them differ by 1.

Constraints

1\leq N,K\leq 100

N and K are integers.

Input

Input is given from Standard Input in the following format:

N K

Output

If we can choose K integers as above, print YES; otherwise, print NO.

Sample Input 1

3 2

Sample Output 1

YES

We can choose 1 and 3.

Sample Input 2

5 5

Sample Output 2

NO

Sample Input 3

31 10

Sample Output 3

YES

Sample Input 4

10 90

Sample Output 4

NO

Solution in C (Clang 3.8.0) (judge verdict: Time Limit Exceeded)

#include <stdio.h>
#define FALSE 0
#define TRUE 1

int dfs(int n, int k){
    int res = FALSE;
    int search = FALSE;
    if(k==0)
        return TRUE;

    else{
        for(int i=2; i+1<=n; i++){
            search += dfs(i, k-1);
        }
        return search + res;
    }
}

int main(){
    int n, k;
    scanf("%d %d", &n, &k);
    if(dfs(n, k-1))
        puts("YES");
    else
        puts("NO");

    return 0;
}