#include <iostream>
#include <fstream>
#include <vector>
#include <utility>
#include <queue>
#define NMAX 30001
#define INF 1e9
using namespace std;

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

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

int BFS(int source,int dest)
{
    q.push({source,0});
    while(!q.empty())
    {
        pair <int,int> f=q.front();
        q.pop();
        vis[source]=1;
        int s=g[f.first].size();
        for(int i=0;i<s;++i)
        {
            if(g[f.first][i].first==dest)
                return g[f.first][i].second+f.second;
            if(vis[g[f.first][i].first]==0)
            {
                g[f.first][i].second+=f.second;
                q.push({g[f.first][i].first,g[f.first][i].second});
            }
        }
    }
}


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

    while(m>0)
    {
        int u,v,w;
        fin>>u>>v>>w;
        if(u>v)
        {
            g[u].push_back({v,-w});
            g[v].push_back({u,w});
        }
        else
        {
            g[u].push_back({v,w});
            g[v].push_back({u,-w});
        }
        --m;
    }

    fout<<BFS(source,dest);
    //fout<<res;

	return 0;
}