#include <fstream>
#include <vector>
#include <utility>
#include <queue>
#define NMAX 30001
using namespace std;

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

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

int BFS(int source,int dest)
{
    q.push(source);
    vis[source]=-1;
    while(!q.empty())
    {
        int f=q.front();
        q.pop();
        for(auto x:g[f])
        {
            if(x.first==dest)
                return vis[dest]+x.second;
            if(vis[x.first]==0)
            {
                vis[x.first]+=vis[f]+x.second;
                q.push(x.first);
            }
        }
    }
}


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

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

    fout<<BFS(source,dest)+1;
    //fout<<res;

	return 0;
}