Problem:
Score : 500 points

Problem Statement

Ken loves ken-ken-pa (Japanese version of hopscotch). Today, he will play it on a directed graph G.
G consists of N vertices numbered 1 to N, and M edges. The i-th edge points from Vertex u_i to Vertex v_i.

First, Ken stands on Vertex S. He wants to reach Vertex T by repeating ken-ken-pa. In one ken-ken-pa, he does the following exactly three times: follow an edge pointing from the vertex on which he is standing.

Determine if he can reach Vertex T by repeating ken-ken-pa. If the answer is yes, find the minimum number of ken-ken-pa needed to reach Vertex T. Note that visiting Vertex T in the middle of a ken-ken-pa does not count as reaching Vertex T by repeating ken-ken-pa.

Constraints

2 \leq N \leq 10^5

0 \leq M \leq \min(10^5, N (N-1))

1 \leq u_i, v_i \leq N(1 \leq i \leq M)

u_i \neq v_i (1 \leq i \leq M)

If i \neq j, (u_i, v_i) \neq (u_j, v_j).

1 \leq S, T \leq N

S \neq T

Input

Input is given from Standard Input in the following format:

N M
u_1 v_1
:
u_M v_M
S T

Output

If Ken cannot reach Vertex T from Vertex S by repeating ken-ken-pa, print -1.
If he can, print the minimum number of ken-ken-pa needed to reach vertex T.

Sample Input 1

4 4
1 2
2 3
3 4
4 1
1 3

Sample Output 1

2

Ken can reach Vertex 3 from Vertex 1 in two ken-ken-pa, as follows: 1 \rightarrow 2 \rightarrow 3 \rightarrow 4 in the first ken-ken-pa, then 4 \rightarrow 1 \rightarrow 2 \rightarrow 3 in the second ken-ken-pa. This is the minimum number of ken-ken-pa needed.

Sample Input 2

3 3
1 2
2 3
3 1
1 2

Sample Output 2

-1

Any number of ken-ken-pa will bring Ken back to Vertex 1, so he cannot reach Vertex 2, though he can pass through it in the middle of a ken-ken-pa.

Sample Input 3

2 0
1 2

Sample Output 3

-1

Vertex S and Vertex T may be disconnected.

Sample Input 4

6 8
1 2
2 3
3 4
4 5
5 1
1 4
1 5
4 6
1 6

Sample Output 4

2

Solution in Go (1.6) (judge verdict: Time Limit Exceeded)

package main

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"math"
	"os"
	"strconv"
)

/********** I/O usage **********/

//str := ReadString()
//i := ReadInt()
//X := ReadIntSlice(n)
//S := ReadRuneSlice()
//a := ReadFloat64()
//A := ReadFloat64Slice(n)

//str := ZeroPaddingRuneSlice(num, 32)
//str := PrintIntsLine(X...)

/*******************************************************************/

const MOD = 1000000000 + 7
const ALPHABET_NUM = 26
const INF_INT64 = math.MaxInt64
const INF_BIT60 = 1 << 60

var n, m int
var U, V []int
var s, t int

var nexts [100000 + 5][]int
var revNexts [100000 + 5][]int

var dp [100000 + 5]int
var flags [100000 + 5][5]bool

type Step struct {
	cid        int
	threeBefId int
	resiStep   int
}

func main() {
	n, m = ReadInt(), ReadInt()
	U, V = make([]int, m), make([]int, m)
	for i := 0; i < m; i++ {
		u, v := ReadInt()-1, ReadInt()-1
		U[i], V[i] = u, v
	}
	s, t = ReadInt()-1, ReadInt()-1

	for i := 0; i < n; i++ {
		nexts[i] = []int{}
		revNexts[i] = []int{}
	}
	for i := 0; i < m; i++ {
		u, v := U[i], V[i]
		nexts[u] = append(nexts[u], v)
		revNexts[v] = append(revNexts[v], u)
	}

	for i := 0; i < n; i++ {
		dp[i] = INF_BIT60
	}
	dp[t] = 0
	flags[t][0] = true

	queue := []Step{}
	for _, v := range revNexts[t] {
		s := Step{cid: v, threeBefId: t, resiStep: 2}
		queue = append(queue, s)
	}

	for len(queue) > 0 {
		s := queue[0]
		queue = queue[1:]

		if s.resiStep == 0 {
			if dp[s.cid] > dp[s.threeBefId]+1 {
				dp[s.cid] = dp[s.threeBefId] + 1
				flags[s.cid][0] = true
				for _, nid := range revNexts[s.cid] {
					if flags[nid][2] {
						continue
					}
					queue = append(queue, Step{cid: nid, threeBefId: s.cid, resiStep: 2})
				}
			}
		} else {
			flags[s.cid][s.resiStep] = true
			for _, nid := range revNexts[s.cid] {
				if flags[nid][s.resiStep-1] {
					continue
				}
				queue = append(queue, Step{cid: nid, threeBefId: s.threeBefId, resiStep: s.resiStep - 1})
			}
		}
	}

	if flags[s][0] {
		fmt.Println(dp[s])
	} else {
		fmt.Println(-1)
	}
}

// func rec(cid, threeBefId, step int) {
// 	// if flags[cid] && step == 0 {
// 	// 	return
// 	// }

// 	if step == 0 {
// 		// ChMin(&dp[cid], dp[threeBefId]+1)
// 		if dp[cid] > dp[threeBefId]+1 {
// 			dp[cid] = dp[threeBefId] + 1
// 			flags[cid] = true
// 			for _, nid := range revNexts[cid] {
// 				rec(nid, cid, 2)
// 			}

// 		}
// 	} else {
// 		for _, nid := range revNexts[cid] {
// 			rec(nid, threeBefId, step-1)
// 		}
// 	}

// 	// 次のノードへ
// 	// for _, nid := range revNexts[cid] {
// 	// 	if step > 0 {
// 	// 		rec(nid, threeBefId, step-1)
// 	// 	} else {
// 	// 		if flags[cid] {
// 	// 			return
// 	// 		}
// 	// 		ChMin(&dp[nid], dp[cid]+dp[threeBefId])
// 	// 		flags[nid] = true
// 	// 		rec(nid, nid, 3)
// 	// 	}
// 	// }
// }

// MODはとったか？
// 遷移だけじゃなくて最後の最後でちゃんと取れよ？

/*******************************************************************/

/*********** I/O ***********/

var (
	// ReadString returns a WORD string.
	ReadString func() string
	stdout     *bufio.Writer
)

func init() {
	ReadString = newReadString(os.Stdin)
	stdout = bufio.NewWriter(os.Stdout)
}

func newReadString(ior io.Reader) func() string {
	r := bufio.NewScanner(ior)
	r.Buffer(make([]byte, 1024), int(1e+11))
	// Split sets the split function for the Scanner. The default split function is ScanLines.
	// Split panics if it is called after scanning has started.
	r.Split(bufio.ScanWords)

	return func() string {
		if !r.Scan() {
			panic("Scan failed")
		}
		return r.Text()
	}
}

// ReadInt returns an integer.
func ReadInt() int {
	return int(readInt64())
}

func readInt64() int64 {
	i, err := strconv.ParseInt(ReadString(), 0, 64)
	if err != nil {
		panic(err.Error())
	}
	return i
}

// ReadIntSlice returns an integer slice that has n integers.
func ReadIntSlice(n int) []int {
	b := make([]int, n)
	for i := 0; i < n; i++ {
		b[i] = ReadInt()
	}
	return b
}

// ReadFloat64 returns an float64.
func ReadFloat64() float64 {
	return float64(readFloat64())
}

func readFloat64() float64 {
	f, err := strconv.ParseFloat(ReadString(), 64)
	if err != nil {
		panic(err.Error())
	}
	return f
}

// ReadFloatSlice returns an float64 slice that has n float64.
func ReadFloat64Slice(n int) []float64 {
	b := make([]float64, n)
	for i := 0; i < n; i++ {
		b[i] = ReadFloat64()
	}
	return b
}

// ReadRuneSlice returns a rune slice.
func ReadRuneSlice() []rune {
	return []rune(ReadString())
}

/*********** Debugging ***********/

// ZeroPaddingRuneSlice returns binary expressions of integer n with zero padding.
// For debugging use.
func ZeroPaddingRuneSlice(n, digitsNum int) []rune {
	sn := fmt.Sprintf("%b", n)

	residualLength := digitsNum - len(sn)
	if residualLength <= 0 {
		return []rune(sn)
	}

	zeros := make([]rune, residualLength)
	for i := 0; i < len(zeros); i++ {
		zeros[i] = '0'
	}

	res := []rune{}
	res = append(res, zeros...)
	res = append(res, []rune(sn)...)

	return res
}

/*********** DP sub-functions ***********/

// ChMin accepts a pointer of integer and a target value.
// If target value is SMALLER than the first argument,
//	then the first argument will be updated by the second argument.
func ChMin(updatedValue *int, target int) bool {
	if *updatedValue > target {
		*updatedValue = target
		return true
	}
	return false
}

// ChMax accepts a pointer of integer and a target value.
// If target value is LARGER than the first argument,
//	then the first argument will be updated by the second argument.
func ChMax(updatedValue *int, target int) bool {
	if *updatedValue < target {
		*updatedValue = target
		return true
	}
	return false
}

// NthBit returns nth bit value of an argument.
// n starts from 0.
func NthBit(num, nth int) int {
	return num >> uint(nth) & 1
}

// OnBit returns the integer that has nth ON bit.
// If an argument has nth ON bit, OnBit returns the argument.
func OnBit(num, nth int) int {
	return num | (1 << uint(nth))
}

// OffBit returns the integer that has nth OFF bit.
// If an argument has nth OFF bit, OffBit returns the argument.
func OffBit(num, nth int) int {
	return num & ^(1 << uint(nth))
}

// PopCount returns the number of ON bit of an argument.
func PopCount(num int) int {
	res := 0

	for i := 0; i < 70; i++ {
		if ((num >> uint(i)) & 1) == 1 {
			res++
		}
	}

	return res
}

/*********** Arithmetic ***********/

// Max returns the max integer among input set.
// This function needs at least 1 argument (no argument causes panic).
func Max(integers ...int) int {
	m := integers[0]
	for i, integer := range integers {
		if i == 0 {
			continue
		}
		if m < integer {
			m = integer
		}
	}
	return m
}

// Min returns the min integer among input set.
// This function needs at least 1 argument (no argument causes panic).
func Min(integers ...int) int {
	m := integers[0]
	for i, integer := range integers {
		if i == 0 {
			continue
		}
		if m > integer {
			m = integer
		}
	}
	return m
}

// DigitSum returns digit sum of a decimal number.
// DigitSum only accept a positive integer.
func DigitSum(n int) int {
	if n < 0 {
		return -1
	}

	res := 0

	for n > 0 {
		res += n % 10
		n /= 10
	}

	return res
}

// Sum returns multiple integers sum.
func Sum(integers ...int) int {
	s := 0

	for _, i := range integers {
		s += i
	}

	return s
}

// CeilInt returns the minimum integer larger than or equal to float(a/b).
func CeilInt(a, b int) int {
	res := a / b
	if a%b > 0 {
		res++
	}
	return res
}

// FloorInt returns the maximum integer smaller than or equal to float(a/b)
func FloorInt(a, b int) int {
	res := a / b
	return res
}

// PowInt is integer version of math.Pow
// PowInt calculate a power by Binary Power (二分累乗法(O(log e))).
func PowInt(a, e int) int {
	if a < 0 || e < 0 {
		panic(errors.New("[argument error]: PowInt does not accept negative integers"))
	}

	if e == 0 {
		return 1
	}

	if e%2 == 0 {
		halfE := e / 2
		half := PowInt(a, halfE)
		return half * half
	}

	return a * PowInt(a, e-1)
}

// AbsInt is integer version of math.Abs
func AbsInt(a int) int {
	if a < 0 {
		return -a
	}
	return a
}

// Gcd returns the Greatest Common Divisor of two natural numbers.
// Gcd only accepts two natural numbers (a, b >= 1).
// 0 or negative number causes panic.
// Gcd uses the Euclidean Algorithm.
func Gcd(a, b int) int {
	if a <= 0 || b <= 0 {
		panic(errors.New("[argument error]: Gcd only accepts two NATURAL numbers"))
	}
	if a < b {
		a, b = b, a
	}

	// Euclidean Algorithm
	for b > 0 {
		div := a % b
		a, b = b, div
	}

	return a
}

// Lcm returns the Least Common Multiple of two natural numbers.
// Lcd only accepts two natural numbers (a, b >= 1).
// 0 or negative number causes panic.
// Lcd uses the Euclidean Algorithm indirectly.
func Lcm(a, b int) int {
	if a <= 0 || b <= 0 {
		panic(errors.New("[argument error]: Gcd only accepts two NATURAL numbers"))
	}

	// a = a'*gcd, b = b'*gcd, a*b = a'*b'*gcd^2
	// a' and b' are relatively prime numbers
	// gcd consists of prime numbers, that are included in a and b
	gcd := Gcd(a, b)

	// not (a * b / gcd), because of reducing a probability of overflow
	return (a / gcd) * b
}

// Strtoi is a wrapper of `strconv.Atoi()`.
// If `strconv.Atoi()` returns an error, Strtoi calls panic.
func Strtoi(s string) int {
	if i, err := strconv.Atoi(s); err != nil {
		panic(errors.New("[argument error]: Strtoi only accepts integer string"))
	} else {
		return i
	}
}

// PrintIntsLine returns integers string delimited by a space.
func PrintIntsLine(A ...int) string {
	res := []rune{}

	for i := 0; i < len(A); i++ {
		str := strconv.Itoa(A[i])
		res = append(res, []rune(str)...)

		if i != len(A)-1 {
			res = append(res, ' ')
		}
	}

	return string(res)
}