#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>

using namespace std;

#define maxn 250010
#define maxk 1010
#define maxd 55

int n, m, k, pz, cost, fiu, nod, a, b, c, x, y, sol[maxn];
vector<int> l[maxk];
vector<pair<int, int> > v[maxn];
char buf[maxd];

int getNr()
{
    int nr=0;
    while(buf[pz]>='0' && buf[pz]<='9')
    {
        nr=nr*10+buf[pz]-'0';
        ++pz;
    }
    ++pz;
    return nr;
}

int main()
{
    freopen("pscnv.in", "r", stdin);
    freopen("pscnv.out", "w", stdout);

    scanf("%d%d%d%d\n", &n, &m, &x, &y);
    for(int i=1; i<=m; ++i)
    {
        fgets(buf, maxd, stdin);
        pz=0;
        a=getNr();
        b=getNr();
        c=getNr();
        v[a].push_back(make_pair(b, c));
    }

    for(int i=1; i<=n; ++i)
        sol[i]=maxk;
    sol[x]=0;

    l[0].push_back(x);
    for(int i=0; i<=1000; ++i)
    {
        for(int pz=0; pz<l[i].size(); ++pz)
        {
            nod=l[i][pz];
            if(nod==y)
            {
                printf("%d\n", i);
                return 0;
            }
            if(sol[nod]!=i)
                continue;
            for(vector<pair<int, int> > :: iterator it = v[nod].begin(); it!=v[nod].end(); ++it)
            {
                fiu=it->first;
                cost=max(i, it->second);
                if(sol[fiu]>cost)
                {
                    sol[fiu]=cost;
                    l[cost].push_back(fiu);
                }
            }
        }
    }
    return 0;
}