#include <stdio.h>
#include <ctype.h>
#define INF 1000000
#define MAXN 250000
#define MAXM 500000
#define MAXK 1000
#define BUF_SIZE 4096
char buf[BUF_SIZE];
int pos=BUF_SIZE;
int lista[MAXN+1], l[MAXK+1], urm[MAXM+1], e[MAXM+1], val[MAXM+1], cost[MAXM+1], next[MAXM+1], ok[MAXN+1], d[MAXN+1], c[MAXK+1];
FILE *fin;
inline int max2(int a, int b){
    if(a>b){
        return a;
    }
    return b;
}
inline char nextch(){
    if(pos==BUF_SIZE){
        fread(buf, BUF_SIZE, 1, fin);
        pos=0;
    }
    return buf[pos++];
}
inline int read(){
    int x=0;
    char ch=nextch();
    while(!isdigit(ch)){
        ch=nextch();
    }
    while(isdigit(ch)){
        x=10*x+ch-'0';
        ch=nextch();
    }
    return x;
}
int main(){
    int n, m, a, b, i, x, k, t, p;
    FILE *fout;
    fin=fopen("pscnv.in", "r");
    fout=fopen("pscnv.out", "w");
    fscanf(fin, "%d%d%d%d", &n, &m, &a, &b);
    for(i=1; i<=m; i++){
        x=read();
        val[i]=read();
        cost[i]=read();
        next[i]=lista[x];
        lista[x]=i;
    }
    for(i=1; i<=n; i++){
        d[i]=INF;
    }
    d[a]=0;
    k=1;
    l[0]=k;
    c[0]=k;
    e[k]=a;
    urm[k]=0;
    t=0;
    while(t<d[b]){
        while(l[t]==0){
            t++;
        }
        if(ok[e[l[t]]]==0){
            ok[e[l[t]]]=1;
            p=lista[e[l[t]]];
            while(p){
                if((ok[val[p]]==0)&&(d[val[p]]>max2(t, cost[p]))){
                    d[val[p]]=max2(t, cost[p]);
                    k++;
                    e[k]=val[p];
                    if(l[d[val[p]]]==0){
                        c[d[val[p]]]=l[d[val[p]]]=k;
                        urm[k]=0;
                    }else{
                        urm[c[d[val[p]]]]=k;
                        c[d[val[p]]]=k;
                    }
                }
                p=next[p];
            }
        }
        l[t]=urm[l[t]];
    }
    fprintf(fout, "%d\n", d[b]);
    fclose(fin);
    fclose(fout);
    return 0;
}