#include <algorithm>
#include <fstream>
#include <queue>
#include <vector>
using namespace std;
const int MAX_N = 30005;
const int UNREACHABLE = 1000000000;
vector< pair<int, int> > distances[MAX_N];
int dist_from_x[MAX_N];

int main() {
    ifstream fin("sate.in");
    ofstream fout("sate.out");
    int n, m, x, y;
    fin >> n >> m >> x >> y;
    for(int i = 1; i <= m; ++ i) {
        int x, y, d;
        fin >> x >> y >> d;
        distances[x].push_back(make_pair(y, d));
        distances[y].push_back(make_pair(x, -d));
    }
    for(int i = 1; i <= n; ++ i) {
        dist_from_x[i] = UNREACHABLE;
    }
    dist_from_x[x] = 0;
    queue<int> q; q.push(x);
    while(!q.empty()) {
        int node = q.front();
        q.pop();
        for(vector< pair<int, int> > :: iterator it = distances[node].begin(); it != distances[node].end(); ++ it) {
            if(dist_from_x[it->first] == UNREACHABLE) {
                dist_from_x[it->first] = dist_from_x[node] + it->second;
                if(it->first == y) {
                    fout << abs(dist_from_x[y]) << "\n";
                    return 0;
                }
                q.push(it->first);
            }
        }
    }
    fout << "-1\n";
    return 0;
}