#include <fstream>
#include <vector>
#include <queue>
using namespace std;
ifstream is ("sate.in");
ofstream os ("sate.out");

queue <int> Q;
vector < pair<int, int> > V[30001];
int n, m, x, y, S;
bool viz[30001];
int d[100025];

void Read();
void BFS();

int main()
{
    Read();
    BFS();
    os << d[y];

    return 0;
}
void Read()
{
    int a, b, c;
    is >> n >> m >> x >> y;
    for(int i = 1; i <= m; ++i)
    {
        is >> a >> b >> c;
        V[a].push_back(make_pair(b, c));
        V[b].push_back(make_pair(a, c));

    }
    Q.push(x);
    viz[x] = true;
}
void BFS()
{
    int nod;
    while(!Q.empty())
    {
        nod = Q.front();
        Q.pop();
        for(unsigned i = 0; i < V[nod].size(); ++i)
        {
            if(!viz[V[nod][i].first])
            {
                if(V[nod][i].first > nod)
                    d[V[nod][i].first] = d[nod] + V[nod][i].second;
                else
                    d[V[nod][i].first] = d[nod] - V[nod][i].second;
                if(V[nod][i].first == y)
                    return;
                Q.push(V[nod][i].first);
                viz[V[nod][i].first] = true;
            }
        }
    }
}