#include<fstream>
#include<queue>
#include<vector>
#define NMAX 30003
#define INF 30000000
using namespace std;
ifstream f("sate.in"); ofstream g("sate.out");
int n, m, s1, s2, nod_a, nod_b, cost, viz[NMAX];
char s[300000];
struct nod
{
    int v;
    int c;
    nod(){};
    nod(int a, int b)
    {
        v =a;
        c = b;
    };
};
queue<int>q;
vector<nod> x[NMAX];
inline void citire();
inline void bfs(int);
int main()
{
    citire();
    bfs(s1);
    g.close();
    return 0;
}
inline void citire()
{
    f>>n>>m>>s1>>s2;
    f.getline(s, 300000);
    for(int i = 1; i <= m; ++i)
    {
        f.getline(s, 300000);
        nod_a = nod_b = cost = 0;
        int j = 0;
        for(j = 0; s[j] != ' '; ++j)
            nod_a = nod_a * 10 + s[j] - '0';
        ++j;
        for(;s[j] != ' '; ++j)
            nod_b = nod_b * 10 + s[j] - '0';
        ++j;
        for(; s[j]; ++j)
            cost = cost * 10 + s[j] - '0';
        x[nod_a].push_back(nod(nod_b, cost));
        x[nod_b].push_back(nod(nod_a, -cost));
        if(i <= n) viz[i] = INF;
    }
    f.close();
}
inline void bfs(int k)
{
    q.push(k);
    viz[k] = 0;
    while(!q.empty())
    {
        k = q.front(); q.pop();
        vector<nod>::iterator it;
        for(it = x[k].begin(); it != x[k].end(); ++it)
        {
            if((*it).v == s2)
            {
                g<<(*it).c + viz[k]<<'\n';
                return;
            }
            viz[(*it).v] = (*it).c + viz[k];
            q.push((*it).v);
        }
    }
}