#include <fstream>
#include <queue>
#include <vector>

using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
#define f first
#define sec second
#define mp make_pair
queue <int> q;
int viz[100001],n,i,m,s[100001],x,y,a,b,d;
vector <pair <int,int> > v[100005];
void bfs(int x)
{
    int i,j;
    q.push(x);
    viz[x]=1;
 while(!q.empty())
    {
        int nod=q.front();
        q.pop();
        for(i=0;i<v[nod].size();i++)
        {
            if(viz[v[nod][i].f]==0)
            {
                q.push(v[nod][i].f);
                s[v[nod][i].f]=s[nod]+v[nod][i].second;//fout<<s[v[nod][i].f]<<" ";
                viz[v[nod][i].f]=1;
                if(v[nod][i].f==b) return;
            }
        }
    }
}
int main()
{
    fin>>n>>m>>a>>b;
    for(i=1;i<=m;i++)
    {
        fin>>x>>y>>d;
        v[x].push_back(mp(y,d));
        v[y].push_back(mp(x,-d));
    }
    bfs(a);
    fout<<s[b];//fout<<"KYS";
    return 0;
}