#include <bits/stdc++.h>
using namespace std;

ifstream in("sate.in");
ofstream out("sate.out");

const int DIM = 1e5 + 57;

int l[DIM];
int r[DIM];
int d[DIM];

int pos[DIM];

int main()
{
	int n, m, x, y;
	in >> n >> m >> x >> y;
	
	for(int i = 1; i <= m; i++)
	{
		in >> l[i] >> r[i] >> d[i];
	}
	
	pos[x] = 1;
	
	while(pos[y] == 0)
	{
		for(int i = 1; i <= m; i++)
			if(pos[l[i]] != 0)
				pos[r[i]] = pos[l[i]] + d[i];
			else
				if(pos[r[i]] != 0)
					pos[l[i]] = pos[r[i]] - d[i];
	}
	
	out << pos[y] - 1;
	
}