#include<cstdio>
#include<cstring>
using namespace std;

constintMaxN=3001;
typedefintArr[MaxN];
int N,M,A,B;
intans;
Arre[MaxN],disa,disb,Lw,L;
voidinit()
{
    scanf("%d%d%d%d",&N,&M,&A,&B);
    memset(L,0,sizeof(L));
    memset(Lw,0,sizeof(Lw));
    for(inti=1;i<=M;++i){
        intx,y; scanf("%d%d",&x,&y);
        L[x]++; L[y]++;
        Lw[x]++; e[x][Lw[x]]=y;
        Lw[y]++; e[y][Lw[y]]=x;
    }
}
voidgetdis(int s,Arr&dis)
{
    memset(dis,0x7F,sizeof(dis));
    inth,t; Arrque;
    h=t=1;
    que[1]=s;
    dis[s]=0;
    while(h<=t){
        intx=que[h++];
        for(inti=1;i<=Lw[x];++i){
            inty=e[x][i];
            if(dis[y]>dis[x]+1){
                dis[y]=dis[x]+1;
                que[++t]=y;
            }
        }
    }
}
voidcalc()
{
    inth,t; Arrque;
    h=1; t=0;
    for(inti=1;i<=N;++i)
        if(L[i]==1)
            que[++t]=i;
    while(h<=t){
        intx=que[h++];
        for(inti=1;i<=Lw[x];++i){
            inty=e[x][i];
            L[y]--;
            if(L[y]==1)
                que[++t]=y;
        }
    }
}
voidsolve()
{
    ans=0;
    for(inti=1;i<=N;++i)
        if(disb[i]>disa[i]&&L[i]>1){
            printf("NIE\n");
            return;
        }
        elseif(disb[i]>ans&&disb[i]>disa[i])
            ans=disb[i];
    printf("%d\n",ans);
}
intmain()
{
    freopen("gon.in","r",stdin);
    freopen("gon.out","w",stdout);
    init();
    getdis(A,disa);
    getdis(B,disb);
    calc();
    solve();
    return0;
}