#include <stdio.h>
#include <vector>
#include <queue>
using namespace std;


vector<vector<pair<int,int> > > a;
unsigned n,st,fi,x,y,aux;
long m,d;
long long dist[30005];

queue<unsigned> q;

void BFS()
{
    for(int i = 0;i<=n;i++)
        dist[i] = -1;

    for(q.push(st);!q.empty();q.pop())
    {
        aux = q.front();
        for(int i = 0;i<a[aux].size();i++)
        {
            if(dist[ a[aux][i].first ] == -1)
            {
                q.push(a[aux][i].first);
                dist[a[aux][i].first] = dist[aux] + a[aux][i].second;
            }
        }
    }
}

int main()
{
    FILE * f = fopen("sate.in","r");
    FILE * g = fopen("sate.out","w");

    fscanf(f,"%d%d%d%d",&n,&m,&st,&fi);

    a.reserve(30005);

    for(int i = 0 ;i<m;i++)
    {
        fscanf(f,"%d%d%d",&x,&y,&d);
        a[x].push_back(make_pair(y,d));
        a[y].push_back(make_pair(x,-d));
    }

    BFS();

    fprintf(g,"%lld",dist[fi]+1);


    fclose(f);
    fclose(g);

    return 0;
}