#include<cstdio>
#include<vector>
using namespace std;
vector<int> v[30001],vec[30001];
int x,y,pp,vc[30001],i,j,n,m,k,l,in=1,sf=1,coada[30001];
int main ()
{
freopen("sate.in","r",stdin);
freopen("sate.out","w",stdout);
scanf("%d%d%d%d",&n,&m,&x,&y);
for(i=1;i<=m;i++)
	{
	int q,qq,qqq;
    scanf("%d%d%d",&q,&qq,&qqq);
    v[q].push_back(qq);
    v[qq].push_back(q);
    vec[q].push_back(qqq);
    vec[qq].push_back(-qqq);
	}
coada[1]=x;
while(in<=sf)
		{
		int nod=coada[in];
		for(int i=0;i<v[nod].size();i++)
			if(vc[v[nod][i]]==0)
				{
				coada[++sf]=v[nod][i];
				vc[v[nod][i]]=vc[nod]+vec[nod][i];
				}
		in++;
		}
printf("%d",vc[y]);
return 0;
}