#include <bits/stdc++.h>
using namespace std;
const int maxK=1e3;
const int maxN=25*1e4+4;
const int INF=0x3f3f3f3f;

class InParser {
private:
    FILE *fin;
    char *buff;
    int sp;

    char read_ch(){
        ++sp;
        if(sp==4096){
            sp=0;
            fread(buff,1,4096,fin);
        }
        return buff[sp];
    }
public:
    InParser(const char* nume){
        fin=fopen(nume,"r");
        buff=new char[4096]();
        sp=4095;
    }

    InParser& operator >> (int &n){
        char c;
        while(!isdigit(c=read_ch())&&c!= '-');
        int sgn=1;
        if(c=='-'){
            n=0;
            sgn=-1;
        }
        else n=c-'0';
        while(isdigit(c=read_ch()))
            n=10*n+c-'0';
        n*=sgn;
        return *this;
    }
};

int dist[maxN];
queue<int> Que[maxK+4];
vector<pair<int,int> >v[maxN];

int n,x,y,m;

int main()
{
    InParser f("pscnv.in");
    freopen("pscnv.out","w",stdout);

    f>>n>>m>>x>>y;

    for(int i=1;i<=m;i++){
        int x,y,z;
        f>>x>>y>>z;
        v[x].push_back(make_pair(y,z));
    }

    for(int i=0;i<=n;i++)
        dist[i]=INF;

    Que[0].push(x);

    int cost=0;
    while(cost<=maxK){
        if(Que[cost].empty()){
            cost++;
            continue;
        }

        int nod=Que[cost].front();
        Que[cost].pop();

        if(dist[nod]==INF){
            if(nod==y){
                printf("%d",cost);
                return 0;
            }

            dist[nod]=cost;
            for(auto it:v[nod])
                if(dist[it.first]==INF)
                    Que[max(cost,it.second)].push(it.first);
        }
    }

    printf("-1");
    return 0;
}