#include <iostream>
#include <vector>
#include <queue>
#include <stdio.h>
#include <string.h>

void bfs(int s, std::vector<std::pair<int, int> > g[], int n, int km[]) {
	
	std::queue<int> q;
	bool visited[n + 1];
	memset(visited, false, sizeof(bool) * (n + 1));
	
	q.push(s);
	visited[s] = true;
	
	while (!q.empty()) {
		
		int node = q.front();
		q.pop();
		
		for (unsigned int i = 0; i < g[node].size(); i++) {
			if (!visited[g[node][i].first]) {
			
				visited[g[node][i].first] = true;
				q.push(g[node][i].first);
				
				if (g[node][i].first < node) {
					km[g[node][i].first] = km[node] - g[node][i].second;
				}
				else {
					km[g[node][i].first] = km[node] + g[node][i].second;
				}
			}
		}
	}
}

int main() {
	
	int n, m, x, y, u, v, cost;
	
	FILE* fin = fopen("sate.in", "r");
	FILE* fout = fopen("sate.out", "w");
	
	fscanf(fin, "%d %d %d %d", &n, &m, &x, &y);
	
	std::vector<std::pair<int, int> > g[n + 1];
	
	
	for (int i = 1; i <= m; i++) {
		fscanf(fin, "%d %d %d", &u, &v, &cost);
		g[u].push_back(std::make_pair(v, cost));
		g[v].push_back(std::make_pair(u, cost));
	}
	
	fclose(fin);
	
	int km[n + 1];
	memset(km, 0, sizeof(int) * (n + 1));
	
	bfs(x, g, n, km);
	fprintf(fout, "%d", km[y]);
	
	fclose(fout);
	
}