#include <fstream>
#include <vector>
#include <queue>
#include <algorithm>
#include <cstring>
using namespace std;

int n,m,x,y,mx,sol,ind;
short ap[250100];
vector< pair<int,short> > A[250100];
vector<int> v[1010];
char buffer[100];

template <class T>
    inline void getnr(T &a)
    {
        a=0;
        while(!(buffer[ind]>='0' && buffer[ind]<='9') && ind<=40)
        {
            ++ind;
        }
        while(buffer[ind]>='0' && buffer[ind]<='9')
        {
            a=a*10+buffer[ind]-'0';
            ++ind;
        }
    }

int main()
{
    ifstream in("pscnv.in");
    ofstream out("pscnv.out");

    int x1,y1,c;
    in>>n>>m>>x>>y;
    in.getline(buffer,40);

    for(int i=1;i<=m;++i)
    {
        ind=0;
        in.getline(buffer,40);
        getnr(x1);
        getnr(y1);
        getnr(c);

        if(mx<c)
        {
            mx=c;
        }
        A[x1].push_back(make_pair(y1,c));
    }

    v[0].push_back(x);

    for(int i=0;i<=mx;++i)
    {
        for(int it=0;it<v[i].size();++it)
        {
            int nod=v[i][it];
            if(nod==y)
            {
                out<<i<<'\n';
                return 0;
            }

            if(!ap[nod])
            {
                ap[nod]=1;
                for(vector< pair<int,short> >::iterator it2=A[nod].begin();it2!=A[nod].end();++it2)
                {
                    int cost=max((short)i,it2->second);
                    v[cost].push_back(it2->first);
                }
            }
        }
    }

    out.close();
    return 0;
}