#include <iostream>
#include <fstream>
#include <vector>
#include <utility>
#include <queue>
#define NMAX 30000
#define INF 1e9
using namespace std;

ifstream fin("sate.in");
ofstream fout("sate.out");

vector <pair <int,int>> g[NMAX];
int n,vis[NMAX];

int DFS(int source,int dest)
{
    vis[source]=1;
    for(auto x:g[source])
        if(vis[x.first]==0)
    {
        if(x.first==dest)
            if(source>dest)
                return -x.second;
            else
                return x.second;
        else
        {
            int c=DFS(x.first,dest);
            if(c!=INF)
                if(source>x.first)
                    return c-x.second;
                else
                    return c+x.second;
        }
    }

    return INF;
}


int main() {
    int m,source,dest;
    fin>>n>>m>>source>>dest;

    while(m--)
    {
        int u,v,w;
        fin>>u>>v>>w;
        g[u].push_back({v,w});
        g[v].push_back({u,w});
    }

    int res=DFS(source,dest);
    fout<<'\n'<<res;

	return 0;
}