Problem:
Score : 100 points

Problem Statement

Three poles stand evenly spaced along a line. Their heights are a, b and c meters, from left to right.
We will call the arrangement of the poles beautiful if the tops of the poles lie on the same line, that is, b-a = c-b.

Determine whether the arrangement of the poles is beautiful.

Constraints

1 \leq a,b,c \leq 100

a, b and c are integers.

Input

Input is given from Standard Input in the following format:

a b c

Output

Print YES if the arrangement of the poles is beautiful; print NO otherwise.

Sample Input 1

2 4 6

Sample Output 1

YES

Since 4-2 = 6-4, this arrangement of poles is beautiful.

Sample Input 2

2 5 6

Sample Output 2

NO

Since 5-2 \neq 6-5, this arrangement of poles is not beautiful.

Sample Input 3

3 2 1

Sample Output 3

YES

Since 1-2 = 2-3, this arrangement of poles is beautiful.

Solution in C++ (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<cstdio>
int n,m,s[200000],a,l,r,mid,ans;
int main()
{
	scanf("%d %d %d",&l,&mid,&r);
	if(mid-l==r-mid) printf("NO");
	else printf("YES");
//	scanf("%d",&n);
//	for(int i=1;i<=n;i++) scanf("%d",&s[i]);
//	scanf("%d",&m);
//	for(int i=1;i<=m;i++){
//		scanf("%d",&a);
//		if(s[1]>a) {printf("%d\n",s[1]);continue;}
//		if(s[n]<a) {printf("%d\n",s[n]);continue;}
//		l=1,r=n;
//		while(l<r){
//			mid=(l+r)>>1;
//			if(s[mid]>=a) r=mid;
//			else l=mid+1;
//		}
//		if(s[l]==a) {printf("%d\n",s[l]);continue;}
//		ans=s[l]-a<a-s[l-1]?s[l]:s[l-1];
//		printf("%d\n",ans);
//	}
	return 0;
}