Problem L

#include<bits/stdc++.h>
#define ll long long int
#define pb push_back
#define mp make_pair
#define pf printf
#define sf scanf
#define ins insert

#define NMax 200002
using namespace std;

vector<int> addj[200010]; // declare the space needed
int vis[200010],aval[200010], bval[200010], abval[200010]; // SIZE

void dfs(int a)
    {
        vis[a]=-1;
        for (int i = 0; i < addj[a].size(); ++i)
        {
            if(vis[addj[a][i]]==-1)
                continue;

            else if (vis[addj[a][i]]==0)
            {
                dfs(addj[a][i]);
            }
            aval[a]=min(aval[a],aval[addj[a][i]]+1);
            bval[a]=min(bval[a],bval[addj[a][i]]+1);
            abval[a]=min(abval[a],abval[addj[a][i]]+1);
        }
        abval[a]=min(abval[a],aval[a]+bval[a]);
        vis[a]=1;
    }

int main()
{
    int n,m,a,b;
    cin>>n>>m>>a>>b;
    int y,u;
    vector<int>v[n+1];

    for(int i=0;i<n+1;i++)
    {
        bval[i]=NMax;
        aval[i]=NMax;
        abval[i]=NMax;
    }
    aval[a]=0;
    bval[b]=0;
    for(int i=0;i<m;i++)
    {
        cin>>y>>u;
        v[y].pb(u);
    }

    for(int i=0;i<n+1;i++)
        addj[i]=v[i];

    dfs(0);

    cout<<abval[0]<<endl;

    return 0;
}