#include<bits/stdc++.h>
using namespace std;
int graph[61][61];
int n,m,s,e,a,b;
int main(){
    freopen("city.in","r",stdin);
    freopen("city.out","w",stdout);
    memset(graph,0x3f,sizeof(graph));
    cin>>n>>m>>s>>e;
    for(int i=0;i<m;i++){
        cin>>a>>b;
        graph[a][b]=1;
        graph[b][a]=1;
    }
    for(int k=1;k<=n;k++){
        for(int i=1;i<=n;i++){
            for(int j=1;j<=n;j++){
                if(graph[i][k]+graph[k][j]<graph[i][j]){
                    graph[i][j]=graph[i][k]+graph[k][j];
                }
            }
        }
    }
    cout<<graph[s][e]+1<<endl;
    return 0;
}