#include <fstream>
#include <vector>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
int n,m,x,y,i,a,b,c,aux,st,dr,k;
int d[30005],q[30005];
bool viz[30005];
vector<pair<int,int> > G[30005];
int main()
{
    fin>>n>>m>>x>>y;
    if(x>y){aux=x;x=y;y=aux;}
    for(i=1;i<=m;i++)
    {
        fin>>a>>b>>c;
        if(a>b){aux=a;a=b;b=aux;}
        G[a].push_back(make_pair(b,c));
        G[b].push_back(make_pair(a,-c));
    }
    st=dr=1;q[1]=x;viz[x]=1;
    while(st<=dr)
    {
        k=q[st];st++;
        //fout<<k<<" "<<d[k]<<"\n";
        for(int j=0;j<G[k].size();j++)
        {
            int vec=G[k][j].first;
            if(!viz[vec])
            {
                viz[vec]=1;
                d[vec]=d[k]+G[k][j].second;
                if(vec==y)
                {
                    fout<<d[vec];
                    return 0;
                }
                q[++dr]=vec;
            }
        }
    }
    fout<<d[y];
    return 0;
}