#include<fstream>
#include<vector>
using namespace std;
ifstream in("sate.in");
ofstream out("sate.out");

const int nmax = 30009;
vector <int> v[nmax], vcost[nmax];
int n, m, x, y; 
long long rasp, act, vdist[nmax];
bool poz[nmax];

void dfs(int x)
{
	poz[x] = 1;
	vdist[x] = act;

	for(int i = 0; i<v[x].size(); i++)
	{
		if(!poz[v[x][i]])
		{
			act = act + vcost[x][i];
			dfs(v[x][i]);
		}
	}
}


int main(){
	int player_unu=0;

	in>>n>>m>>x>>y;
	for(int i = 0; i<m; i++)
	{
		int a, b, cost;
		in>>a>>b>>cost;

		v[a].push_back(b);
		v[b].push_back(a);

		vcost[a].push_back(cost);
		vcost[b].push_back(-cost);
	}

	dfs(x);

	out<<vdist[y] - vdist[x];
	return player_unu;
}