#include<fstream>
#include<vector>
#include<queue>
#define Nmax 30001
#define pb push_back
using namespace std;
vector< pair<int,int> >G[Nmax];
queue<int>q;
int d[Nmax], viz[Nmax], i, j, n, m, a, b, cost;
int start, stop;

void drum()
{
	viz[start]=1;
	q.push(start);
	while(!q.empty())
	{
		a=q.front();
		q.pop();
		for(i=0;i<G[a].size();i++)
			if(!viz[G[a][i].first])
			{
				viz[G[a][i].first]=1;
				if(G[a][i].first>a)
					d[G[a][i].first]=d[a]+G[a][i].second;
				else
					d[G[a][i].first]=d[a]-G[a][i].second;
				q.push(G[a][i].first);
			}
	}
}	

int main()
{
	freopen("sate.in", "r", stdin);
	freopen("sate.out", "w", stdout);
	scanf("%d%d%d%d", &n, &m, &start, &stop);
	for(i=1;i<=m;i++)
	{
		scanf("%d%d%d", &a, &b, &cost);
		G[a].pb(make_pair(b, cost));
		G[b].pb(make_pair(a, cost));
	}
	drum();
	printf("%d\n", d[stop]);
	fclose(stdin);
	fclose(stdout);
	return 0;
}