#include <iostream>
#include<stdio.h>
#include<stdlib.h>
//#include<limits.h>

using namespace std;

struct Queue
{
    int font,rear,siz;
    unsigned capacity;
    int* arr;
};

struct Queue* create(unsigned capacity)
{
    struct Queue* que = (struct Queue*) malloc(sizeof(struct Queue));
    que->font=que->siz=0;
    que->rear=capacity-1;
    que->capacity=capacity;
    que->arr = (int*) malloc(que->capacity * sizeof(int));
    return que;
};

int isFull(struct Queue* que)
{
    if(que->siz==que->capacity) return 1;
    else return 0;
}

int isEmpty(struct Queue* que)
{
    if(que->siz==0) return 1;
    else return 0;
}

void push(struct Queue* que, int item)
{
    if(isFull(que)) return;

    que->rear = (que->rear+1)%que->capacity;

    que->arr[que->rear]=item;
    que->siz= que->siz + 1;

}

void pop(struct Queue* que)
{
    if(isEmpty(que)) return;

    que->font= (que->font + 1)%que->capacity;
    que->siz = que->siz -1;

}

int font (struct Queue* que)
{
    if (isEmpty(que)) return INT_MIN;
    return que->arr[que->font];
}

int rear (struct Queue* que)
{
    if (isEmpty(que)) return INT_MIN;
    return que->arr[que->rear];
}

int visited[100];
int mat[100][100];

int main()
{



    for(int i=0; i<100; i++) visited[i]=-1;

    int n,e;
    cin>>n>>e;

    for(int i=1; i<=e; i++)
    {
        int a,b;
        cin>>a>>b;
        mat[a][b]=1;
        mat[b][a]=1;
    }


    int src,des;

    cin>>src>>des;

    struct Queue* que = create(100);

    push(que,src);
    visited[src]=0;

    while(!isEmpty(que))
    {
        int p=font(que);
        pop(que);

        for(int i=1; i<=n; i++ )
        {
            int t=i;

            if(visited[t]==-1 && mat[p][t]==1 and t!=p)
            {
                visited[t]=visited[p]+1;
                push(que,t);

                if(t==des)
                {
                    cout<<"reachable" << " :"<<visited[t];
                    break;
                }
            }
        }

    }
}