#include<iostream>
#include<vector>
#include<queue>
using namespace std;
int main(){
    vector<int>a[5001];
    int n,m,s,t,in[5001],x,y;
    long long dp[5001]={0};
    cin>>n>>m>>s>>t;
    dp[s]=1;
    while(m--){
        cin>>x>>y;
        a[x].push_back(y);
        in[y]++;
    }queue<int>q;
    for(int i=1;i<=n;i++)if(!in[i])q.push(i);
    while(q.size()){
        for(int i=0;i<a[q.front()].size();i++){
            if(--in[a[q.front()][i]]==0)q.push(a[q.front()][i]);
            dp[a[q.front()][i]]+=dp[q.front()];
        }q.pop();
    }if(dp[t])cout<<dp[t];
    else cout<<"No Path!";
    return 0;
}