#include <stdio.h>
#include <string.h>
#include <vector>

using namespace std;

#define NMAX 25002
#define INFI 0x3f3f3f3f

int n, m, x, y;

#define DIM 4000000
char buf[DIM];
int poz;
#define cin(x)		\
{			\
	x = 0;		\
	while(buf[poz] < '0' || buf[poz] > '9')  \
		if(++poz == DIM)		\
			fread(buf, 1, DIM, stdin), poz = 0; 	\
	while(buf[poz] >= '0' && buf[poz] <= '9')		\
	{							\
		x = x*10 + (buf[poz]-'0');			\
		if(++poz == DIM)				\
			fread(buf, 1, DIM, stdin), poz = 0;	\
	}							\
}

#define MAX(a, b) ((a) > (b)) ? (a) : (b)
#define MIN(a, b) ((a) < (b)) ? (a) : (b)

vector<int> a[NMAX], p[NMAX], k[1002];
int _max = -INFI, _min[NMAX];

void read()
{
	fread(buf, 1, DIM, stdin);
	cin(n); cin(m); cin(x); cin(y);
    //scanf("%d%d%d%d", &n, &m, &x, &y);
	for(int i = 1, q, w, e; i <= m; ++i)
	{
		cin(q); cin(w); cin(e);
        //scanf("%d%d%d", &q, &w, &e);
		a[q].push_back(w);
		p[q].push_back(e);
		_max = MAX(_max, e);
	}
}	



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

	read();

	memset(_min, INFI, sizeof(_min));
	_min[x] = 0;

	int i;
	vector<int> :: iterator nod, cost, fin1, fin2, it;
	
	k[0].push_back(x);

	for(i = 0; i <= _max; ++i)
	{
		for(it = k[i].begin(), fin1 = k[i].end(); it != fin1; ++it)
		{
			if(_min[*it] < i)
				continue;

			for(nod = a[*it].begin(), cost = p[*it].begin(), fin2 = a[*it].end(); nod != fin2; ++nod, ++cost)
			{
				_min[*nod] = MIN(_min[*nod], MAX(*cost, _min[*it]));
				k[ _min[*nod] ].push_back(*nod);
			}
		}
	}

	printf("%d\n", _min[y]);

	return 0;
}