#include<fstream>
#include<vector>
#include<queue>
#include<bitset>
std::ifstream fin("sate.in");
std::ofstream fout("sate.out");
const int NMAX=30005;
std::vector<std::pair<int, int>>G[NMAX];//dist, to
std::bitset<NMAX>f;
int n, m, x, y;
void read()
{
    fin>>n>>m>>x>>y;
    for(int i=0; i<m; ++i)
    {
        int from, to, dist;
        fin>>from>>to>>dist;
        G[from].emplace_back(dist, to);
        G[to].emplace_back(dist, from);
    }
}
int bfs()
{
    std::queue<std::pair<int, int>>q;//current_value, node
    q.emplace(0, x);
    f[x]=true;
    while(!q.empty())
    {
        std::pair<int, int>local=q.front();
        q.pop();
        for(auto it:G[local.second])
        {
            if(!f[it.second])
            {
                f[it.second]=true;

                int newCost=local.first;
                if(it.second<local.second)
                    newCost-=it.first;
                else
                    newCost+=it.first;

                if(it.second==y)
                    return newCost;
                q.emplace(newCost, it.second);
            }
        }
    }
    return -1;
}
int main()
{
    std::ios_base::sync_with_stdio(false);
    read();
    fout<<bfs();
    return 0;
}