#include <fstream>
#include <vector>
#include <deque>
#define DIM 30001
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
int n,m,a,b,x,y,c,d[DIM];
vector<pair<int,int>> l[DIM];
deque<int> q;

void bfs(int p) {
    q.push_back(p);
    d[p]=0;
    while (!q.empty()) {
        int nod=q.front();
        q.pop_front();
        for (int i=0;i<l[nod].size();i++) {
            int vec=l[nod][i].first;
            int cost=l[nod][i].second;
            if (d[vec]==-1) {
                if (vec>nod)
                    d[vec]=d[nod]+cost;
                else
                    d[vec]=d[nod]-cost;
                q.push_back(vec);
            }
        }
    }
}

int main() {
    fin>>n>>m>>a>>b;
    while (m--) {
        fin>>x>>y>>c;
        l[x].push_back({y,c});
        l[y].push_back({x,c});
    }
    for (int i=1;i<=n;i++)
        d[i]=-1;
    bfs(a);
    fout<<d[b];
    return 0;
}