#include <cstdio>
#include <queue>
#include <vector>
using namespace std;
struct Muchie
{
    int y,z;
};
vector<Muchie> g[30005];
int dist[30005];
bool viz[30005];
queue<int> q;
int bfs(int start, int finish)
{
    q.push(start);
    dist[start]=0;
    viz[start]=1;
    while(!q.empty()){
        int u=q.front();
        q.pop();
        for(int i=0; i<g[u].size(); i++)
            if(viz[g[u][i].y]==0){
                if(u>g[u][i].y)
                    dist[g[u][i].y]=dist[u]-g[u][i].z;
                else
                    dist[g[u][i].y]=dist[u]+g[u][i].z;
                viz[g[u][i].y]=1;
                q.push(g[u][i].y);
            }
    }
    return dist[finish];
}
int main()
{   freopen("sate.in", "r", stdin);
    freopen("sate.out", "w", stdout);
    int n,m,x,y,x1,y1,z,i;
    scanf("%d%d%d%d", &n, &m, &x, &y);
    for(i=1; i<=m; i++){
        scanf("%d%d%d", &x1, &y1, &z);
        g[x1].push_back({y1, z});
        g[y1].push_back({x1, z});
    }
    printf("%d", bfs(x, y));
    return 0;
}