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 (1.6) (judge verdict: Wrong Answer)

package main

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

var (
	rdr   = bufio.NewReaderSize(os.Stdin, 1000000)
	reply string
)

func main() {
	in := nextSlice()
	solve(in)
}

func solve(buf []string) {
	a, _ := strconv.Atoi(buf[0])
	b, _ := strconv.Atoi(buf[1])
	c, _ := strconv.Atoi(buf[2])
	if b-a == c-b {
		yesorNO(true)
	} else {
		yesorNO(false)
	}
}

func readLine() string {
	buf := make([]byte, 0, 1000000)
	for {
		l, p, e := rdr.ReadLine()
		if e != nil {
			panic(e)
		}
		buf = append(buf, l...)
		if !p {
			break
		}
	}
	return string(buf)
}

func nextSlice() []string {
	return strings.Split(readLine(), " ")
}

func atoi(a []string) (b []int) {
	b = make([]int, len(a))
	for i := range a {
		b[i], _ = strconv.Atoi(a[i])
	}
	return
}

func atof(a []string) (b []float64) {
	b = make([]float64, len(a))
	for i := range a {
		b[i], _ = strconv.ParseFloat(a[i], 64)
	}
	return
}

func scanSlice2(n int) (a []int, b []int) {
	a = make([]int, n)
	b = make([]int, n)
	for i := 0; i < n; i++ {
		fmt.Scan(&a[i], &b[i])
	}
	return
}

func scanFloatSlice2(n int) (a []float64, b []float64) {
	a = make([]float64, n)
	b = make([]float64, n)
	for i := 0; i < n; i++ {
		fmt.Scan(&a[i], &b[i])
	}
	return
}

// ---------------------------------------------------

func abs(a int) int {
	if a > 0 {
		return a
	}
	return -a
}

func gcd(a, b int) int {
	if b == 0 {
		return a
	}
	return gcd(b, a%b)
}

func min(a ...int) int {
	min := a[0]
	for i := 1; i < len(a); i++ {
		if min > a[i] {
			min = a[i]
		}
	}
	return min
}

func max(a ...int) int {
	max := a[0]
	for i := 1; i < len(a); i++ {
		if max < a[i] {
			max = a[i]
		}
	}
	return max
}

func sum(a []int) (ans int) {
	for i := range a {
		ans += a[i]
	}
	return
}

// ---------------------------------------------------

func yesOrNo(b bool) {
	if b {
		reply = "Yes"
		fmt.Println(reply)
		return
	}
	reply = "No"
	fmt.Println(reply)
}

func yesorNO(b bool) {
	if b {
		reply = "Yes"
		fmt.Println(reply)
		return
	}
	reply = "No"
	fmt.Println(reply)
}