#pragma GCC optimize ("O3")

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <queue>
#include <set>
#include <algorithm>
#include <numeric>
#include <list>
using namespace std;

using QWORD  = uint64_t;
using SQWORD = int64_t;
using DWORD  = uint32_t;
using SDWORD = int32_t;
using WORD   = uint16_t;
using SWORD  = int16_t;
using BYTE   = uint8_t;
using SBYTE  = int8_t;
using DOUBLE = double;
using FLOAT  = float;

#define MIN_SDWORD (-2147483648)
#define MAX_SDWORD (2147483647)
#define MIN_SBYTE (-128)
#define MAX_SBYTE (127)

#define MIN_SQWORD (0x8000000000000000)
#define MAX_SQWORD (0x7FFFFFFFFFFFFFFF)

#define MAX_QWORD  (0xFFFFFFFFFFFFFFFF)
#define MAX_DWORD  (0xFFFFFFFF)
#define MAX_WORD   (0xFFFF)
#define MAX_BYTE   (0xFF)


#define ArrayLength(a)  (sizeof(a) / sizeof(a[0]))

static inline QWORD MAX(QWORD a, QWORD b) { return a > b ? a : b; }
static inline DWORD MAX(DWORD a, DWORD b) { return a > b ? a : b; }
static inline SDWORD MAX(SDWORD a, SDWORD b) { return a > b ? a : b; }
static inline QWORD MIN(QWORD a, QWORD b) { return a < b ? a : b; }
static inline DWORD MIN(DWORD a, DWORD b) { return a < b ? a : b; }
static inline SDWORD MIN(SDWORD a, SDWORD b) { return a < b ? a : b; }

#define BYTE_BITS   (8)
#define WORD_BITS   (16)
#define DWORD_BITS  (32)
#define QWORD_BITS  (64)

using M_BOOL = bool;
#define M_TRUE (true)
#define M_FALSE (false)
#define DIVISOR (1000000007)

static inline void inputString(char *pcStr)
{
    char *pcCur = pcStr;
    for (;;) {
        char c = getchar();
        if (('\n' == c) || (EOF == c)) {
            break;
        }
        *pcCur = c;
        pcCur++;
    }
    *pcCur = '\0';
}


static inline SQWORD inputSQWORD(void)
{
    SQWORD sqNumber = 0;
    SQWORD sqMultiplier = 1;
    M_BOOL bRead = M_FALSE;
    for (;;) {
        char c = getchar();
        if (!bRead) {
            if ('-' == c) {
                sqMultiplier = -1;
            }
        }
        if (('0' <= c) && (c <= '9')) {
            sqNumber *= 10LL;
            sqNumber += (SQWORD)(c - '0');
            bRead = M_TRUE;
        } else {
            if (bRead) {
                return sqNumber * sqMultiplier;
            }
        }
    }
}


static inline SDWORD inputSDWORD(void)
{
    SDWORD lNumber = 0;
    SDWORD lMultiplier = 1;
    M_BOOL bRead = M_FALSE;
    for (;;) {
        char c = getchar();
        if (!bRead) {
            if ('-' == c) {
                lMultiplier = -1;
            }
        }
        if (('0' <= c) && (c <= '9')) {
            lNumber *= 10;
            lNumber += (c - '0');
            bRead = M_TRUE;
        } else {
            if (bRead) {
                return lNumber * lMultiplier;
            }
        }
    }
}

static inline DOUBLE inputFP(void)
{
    DOUBLE dInt = 0.0;
    DOUBLE dFrac = 0.0;
    DOUBLE dMultiplier = 1.0;
    DWORD dwFpCnt = 0;
    DOUBLE *pdCur = &dInt;
    M_BOOL bRead = M_FALSE;
    for (;;) {
        char c = getchar();
        if (!bRead) {
            if ('-' == c) {
                dMultiplier = -1;
            }
        }
        if ('.' == c) {
            pdCur = &dFrac;
        } else if (('0' <= c) && (c <= '9')) {
            (*pdCur) *= 10;
            (*pdCur) += (DOUBLE)(c - '0');
            bRead = M_TRUE;
            if (pdCur == &dFrac) {
                dwFpCnt++;
            }
        } else {
            if (bRead) {
                return dMultiplier * (dInt + dFrac / (pow((DOUBLE)10.0 , (DOUBLE)dwFpCnt)));
            }
        }
    }
}


/**
 *  mod による操作ライブラリ
 */

#define ANS_MOD (1000000007LL)
 
static SQWORD addMod(SQWORD x, SQWORD y)
{ 
    return (x + y) % ANS_MOD;
}
 
static SQWORD subMod(SQWORD x, SQWORD y)
{
    return (x - y + ANS_MOD) % ANS_MOD;
}
 
static SQWORD mulMod(SQWORD x, SQWORD y) 
{
    return (x * y) % ANS_MOD;
}
 
static SQWORD powMod(SQWORD x, SQWORD e) {
    SQWORD v = 1;
    for (; e; x = mulMod(x, x), e >>= 1) {
        if (e & 1) {
            v = mulMod(v, x);
        }
    }
    return v;
}
 
static SQWORD divMod(SQWORD x, SQWORD y)
{
    return mulMod(x, powMod(y, ANS_MOD - 2));
}
 
 
static SQWORD combMod(SQWORD n, SQWORD k)
{
    SQWORD v=1;
    for(SQWORD i=1; i<=k; i++) {
        v = divMod(mulMod(v, n-i+1),i);
    } 
    return v;
}

/*----------------------------------------------*/

#define MAX_V           (100000)
#define SQWORD_INF      (1000000000000000000)

struct EDGE_ST {
    SQWORD sqTo;
    SQWORD sqCost;

    EDGE_ST(SQWORD to, SQWORD cost) {
        sqTo = to;
        sqCost = cost;
    };
};

void getNextPnts(SDWORD lStart, const vector<EDGE_ST> *pvstEdges, set<SQWORD> &setNextPnts)
{
    for (auto edge1: pvstEdges[lStart]) {
        for (auto edge2: pvstEdges[edge1.sqTo]) {
            for (auto edge3: pvstEdges[edge2.sqTo]) {
//                printf("-->%lld\n", edge3.sqTo);
                setNextPnts.insert(edge3.sqTo);
            }
        }
    }
}

#define NUM_HOPS        (3)
#define MAX_PNTS        (100000)

int main()
{
    SQWORD sqInput_N = inputSQWORD();       /* number of islands */
    SQWORD sqInput_M = inputSQWORD();       /* number of queries */
    vector<EDGE_ST> vstEdges[sqInput_N + 1];

    for (SQWORD sqIdx = 0; sqIdx < sqInput_M; sqIdx++) {
        SQWORD sq_u = inputSQWORD();
        SQWORD sq_v = inputSQWORD();
        vstEdges[sq_u].emplace_back(EDGE_ST(sq_v, 1));
    }

    SQWORD sqInput_S = inputSQWORD();
    SQWORD sqInput_T = inputSQWORD();

    queue<pair<SQWORD, SQWORD>> quePnt;

    static SQWORD   s_aasqDist[MAX_PNTS + 1][NUM_HOPS];
    for (SDWORD lIdx = 0; lIdx < ArrayLength(s_aasqDist); lIdx++) {
        for (SDWORD lDim = 0; lDim < ArrayLength(s_aasqDist[0]); lDim++) {
            s_aasqDist[lIdx][lDim] = SQWORD_INF;
        }
    }

    quePnt.push(make_pair(sqInput_S, 0));
    s_aasqDist[sqInput_S][0] = 0;
    while (!(quePnt.empty())) {
        SQWORD sqV = quePnt.front().first;
        SQWORD sqDim  = quePnt.front().second;
        quePnt.pop();

        for (auto next: vstEdges[sqV]) {
            SDWORD lNextPhase = (sqDim + 1) % NUM_HOPS;
            if (SQWORD_INF == s_aasqDist[next.sqTo][lNextPhase]) {
                s_aasqDist[next.sqTo][lNextPhase] = s_aasqDist[sqV][sqDim] + 1;
                quePnt.push(make_pair(next.sqTo, lNextPhase));
            }
        }
    }

    SQWORD sqDist = s_aasqDist[sqInput_T][0];
    if (SQWORD_INF == sqDist) {
        printf("-1\n");
    } else {
        printf("%lld\n", sqDist / 3);
    }
    
    return 0;
}



Measured memory_bytes: 8064