#include <iostream>
#include <fstream>
#include <string>
#include <cstring>
#include <algorithm>
#include <utility>
#include <cmath>
#include <map>
#include <deque>
#include <vector>
#include <set>
#include <queue>
#include <bitset>
#include <limits.h>
using namespace std;

ifstream fin("sate.in");
ofstream fout("sate.out");

const int MAX_SIZE = 30000;

int noVillages, noRoutes, start, finish;
vector<pair<int, int>> routes[MAX_SIZE + 1];
int distances[MAX_SIZE + 1];

void calcDist() {
    for (int i = 1; i <= noVillages; ++i) {
        distances[i] = INT_MAX;
    }
    distances[start] = 0;
    queue<pair<int, int>> villages;
    villages.push(make_pair(start, 0)); // dist cur = 0;
    int ok = 0;
    while (!villages.empty()) {
        int actVillage = villages.front().first;
        int actDistance = villages.front().second;
        if (actVillage == finish) {
            fout << actDistance;
            return;
        }
        villages.pop();
        for (pair<int, int> next : routes[actVillage]) {
            int nextVillage = next.first;
            int nextDistance = next.second;
            if (distances[nextVillage] == INT_MAX) {
                int totalDist = actDistance;
                if (actVillage < nextVillage) { // -> pozitiv
                    totalDist += nextDistance;
                } else {
                    totalDist -= nextDistance;
                }
                distances[nextVillage] = totalDist;
                villages.push(make_pair(nextVillage, totalDist));
            }
        }
    }
    fout << -1;
}

int main() {
    ios_base::sync_with_stdio(false);
    fin >> noVillages >> noRoutes >> start >> finish;
    for (int i = 1; i <= noRoutes; ++i) {
        int left, right, dist;
        fin >> left >> right >> dist;
        routes[left].push_back(make_pair(right, dist));
        routes[right].push_back(make_pair(left, dist));
    }
    calcDist();
}