#include <fstream>
#include <vector>
#define Max 100100
using namespace std;
  
FILE *fin = fopen("sate.in", "r");
FILE *fout = fopen("sate.out", "w");
  
int n, m, i, j, k, ok, minim, maxim,z;
vector < int > v[Max];
int vizit[Max], dist[Max], p, u, x, y;
  
void bfs(int val){
    vizit[val] = 1;
    if(dist[u] != 0){
        fprintf(fout, "%d", dist[u]);
        ok = 1;
    }
    if(ok == 1)
        return;
    for(int i = 0; i < v[val].size(); i += 2){
        if(vizit[v[val][i]] == 0){
            dist[v[val][i]] = dist[val] + v[val][i + 1];
            bfs(v[val][i]);
        }
    }
    return;
}
  
void read(){
    fscanf(fin, "%d%d%d%d", &n, &m, &p, &u);
    for(i = 1; i <= m; i ++){
        fscanf(fin, "%d%d%d", &x, &y, &z);
        v[x].push_back(y);
        v[x].push_back(z);
        v[y].push_back(x);
        v[y].push_back(-z);
    }
    return;
}
  
int main(){
    read(); bfs(p);
    if(ok == 0)
		fprintf(fout, "%d", dist[u]);
    return 0;
}