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 Go (1.14.1) (judge verdict: Wrong Answer)

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

var sc = bufio.NewScanner(os.Stdin)

func Scan() string {
	sc.Scan()
	return sc.Text()
}
func iScan() int {
	n, _ := strconv.Atoi(Scan())
	return n
}
func fScan() float64 {
	n, _ := strconv.ParseFloat(Scan(), 64)
	return n
}
func stringToInt(s string) int {
	n, _ := strconv.Atoi(s)
	return n
}
func SScan(n int) []string {
	a := make([]string, n)
	for i := 0; i < n; i++ {
		a[i] = Scan()
	}
	return a
}
func iSScan(n int) []int {
	a := make([]int, n)
	for i := 0; i < n; i++ {
		a[i] = iScan()
	}
	return a
}
func abs(x int) int {
	if x < 0 {
		x = -x
	}
	return x
}
func larger(a, b int) int {
	if a < b {
		return b
	} else {
		return a
	}
}
func smaller(a, b int) int {
	if a > b {
		return b
	} else {
		return a
	}
}
func max(a []int) int {
	x := a[0]
	for i := 0; i < len(a); i++ {
		if x < a[i] {
			x = a[i]
		}
	}
	return x
}
func min(a []int) int {
	x := a[0]
	for i := 0; i < len(a); i++ {
		if x > a[i] {
			x = a[i]
		}
	}
	return x
}
func sum(a []int) int {
	x := 0
	for _, v := range a {
		x += v
	}
	return x
}

// UnionFind 構造体 for tree
type UnionFind struct {
	par []int
}

/*コンストラクタ*/
func newUnionFind(N int) *UnionFind {
	u := new(UnionFind)
	u.par = make([]int, N)
	for i := range u.par {
		u.par[i] = -1
	}
	return u
}
func (u UnionFind) root(x int) int {
	if u.par[x] < 0 {
		return x
	}
	u.par[x] = u.root(u.par[x])
	return u.par[x]
}
func (u UnionFind) unite(x int, y int) {
	xr := u.root(x)
	yr := u.root(y)
	if xr == yr {
		return
	}
	u.par[yr] += u.par[xr]
	u.par[xr] = yr
}
func (u UnionFind) same(x, y int) bool {
	if u.root(x) == u.root(y) {
		return true
	}
	return false
}
func (u UnionFind) size(x int) int {
	return -u.par[u.root(x)]
}

var mod int = 1000000007

type pair struct {
	fi int
	se int
}

func main() {
	buf := make([]byte, 0)
	sc.Buffer(buf, mod)
	sc.Split(bufio.ScanWords)
	//予備
	a := iScan()
	b := iScan()
	c := iScan()

	if b-a == c-a {
		fmt.Println("YES")
	} else {
		fmt.Println("NO")
	}
}