/* 
 * Problem: POI1998 gon
 * Author: Guo Jiabao
 * Time: 2008.12.3 18:42:14
 * State: Solved 
*/
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>

using namespace std;

class tList
{
	public:
		tList *next;
		int v;
		tList(int tv) : v(tv)
		{
			next=0;
		}
};

class tQueue
{
	public:
		tList *first,*last;
		int size;
		tQueue()
		{
			reset();
		}
		void reset()
		{
			first=last=0;
			size=0;
		}
		void ins(int v)
		{
			if (first)
				last=last->next=new tList(v);
			else
				first=last=new tList(v);
			size++;
		}
		int pop()
		{
			int rtn=first->v;
			tList *t=first;
			size--;
			first=first->next;
			delete t;
			return rtn;
		}
};

const int MAXN=3001;

int N,M,A,B,Ans;
int Sf[MAXN],Degree[MAXN];
int DA[MAXN],DB[MAXN];
tQueue adjl[MAXN],Q;

void init()
{
	int i,a,b;
	freopen("gon.in","r",stdin);
	freopen("gon.out","w",stdout);
	scanf("%d%d%d%d",&N,&M,&A,&B);
	for (i=1;i<=M;i++)
	{
		scanf("%d%d",&a,&b);
		adjl[a].ins(b);
		adjl[b].ins(a);
		Degree[a]++;
		Degree[b]++;
	}
	for (i=1;i<=N;i++)
	{
		Sf[i]=true;
		DA[i]=DB[i]=-1;
	}	
}

void safe()
{
	int i,j;
	tList *k;
	for (i=1;i<=N;i++)
	{
		if (Degree[i]==1)
		{
			Q.ins(i);
		}
	}
	while (Q.size)
	{
		i=Q.pop();
		Degree[i]=0;
		Sf[i]=false;
		for (k=adjl[i].first;k;k=k->next)
		{
			j=k->v;
			Degree[j]--;
			if (Degree[j]==1)
				Q.ins(j);
		}
	}
}

void dist()
{
	int i,j;
	tList *k;
	Q.reset();
	Q.ins(A);
	DA[A]=0;
	while (Q.size)
	{
		i=Q.pop();
		for (k=adjl[i].first;k;k=k->next)
		{
			j=k->v;
			if (DA[j]==-1)
			{
				DA[j]=DA[i]+1;
				Q.ins(j);
			}
		}
	}
	Q.reset();
	Q.ins(B);
	DB[B]=0;
	while (Q.size)
	{
		i=Q.pop();
		for (k=adjl[i].first;k;k=k->next)
		{
			j=k->v;
			if (DB[j]==-1)
			{
				DB[j]=DB[i]+1;
				Q.ins(j);
			}
		}
	}
}

void solve()
{
	int k;
	for (k=1;k<=N;k++)
	{
		if (Sf[k] && DA[k] < DB[k])
		{
			printf("NIE");
			return;
		}
		if (DA[k] < DB[k] && DB[k] > Ans)
		{
			Ans=DB[k];
		}
	}
	printf("%d",Ans);
}

int main()
{
	init();
	safe();
	dist();
	solve();
	return 0;
}