#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
#define  nmax 30002

using namespace std;

const char iname[] = "sate.in";
const char oname[] = "sate.out";

ifstream fin(iname);
ofstream fout(oname);

int n, m, x, y, a, b, c, i;
vector <pair <int, int> > Graf[nmax];
queue<int> Q;
int viz[nmax];
int D[nmax];

void BF()
{
	Q.push(x);
	while(!Q.empty())
	{	
		x = Q.front();
		Q.pop();
		for(int i = 0; i < Graf[x].size(); i ++)
		{
			if(viz[Graf[x][i].first] == 0)
			{
				viz[Graf[x][i].first] = 1;
				D[Graf[x][i].first] = D[x] + Graf[x][i].second;
				Q.push(Graf[x][i].first);
			}
		}
	}
}
	


int main()
{
	fin >> n >> m >> x >> y;
	for(i = 1; i <= m; i ++)
	{
		fin >> a >> b >> c;
		if(b < a)
			swap(a, b);
		Graf[a].push_back(make_pair(b, c));
		Graf[b].push_back(make_pair(a, -c));
	}
	
	BF();
	fout << D[y];
	return 0;
}