#include <cstdio>
#include <vector>
#include <queue>
using namespace std;
const int MAX = 300001, INF = 20000001;
vector < pair <int, int> > t[MAX];
queue <int> q;
int n, m, x, y, d[MAX];
int main()
{
    freopen("sate.in","r",stdin);
    freopen("sate.out","w",stdout);
    scanf("%d%d%d%d", &n, &m, &x, &y);
    for(int i=1; i<=m; i++)
    {
        int a, b, d;
        scanf("%d%d%d", &a, &b, &d);
        t[a].push_back(make_pair(b, d));
        t[b].push_back({a, -d});
    }
    q.push(x);
    d[x] = 0;
    while(!q.empty()){
        int id = q.front();
        q.pop();
        for(unsigned i=0; i<t[id].size(); i++)
        {
            int nextid = t[id][i].first;
            int dist = t[id][i].second;
            if(d[nextid]==0)
            {
                d[nextid] = d[id] + dist;
                q.push(nextid);
            }
        }
    }
    printf("%d", d[y]);
    return 0;
}