#include <iostream>
#include <set>
#include <vector>
using namespace std;

	const int INF = 1000000000;

	int main() {
		int n, m ,s, f;
		cin >> n>>m>>s>>f;
		
		vector < vector < pair<int, int> > > g(n+1);
		// ÷òåíèå ãðàôà
    

		vector<int> d(n+1, INF);
		d[s] = 0;
		set < pair<int, int> > q;
		q.insert(make_pair(d[s], s));
		while (!q.empty()) {
			int v = q.begin()->second;
			q.erase(q.begin());

			for (size_t j = 0; j < g[v].size(); ++j) {
				int to = g[v][j].first,
					len = g[v][j].second;
				if (d[v] + len < d[to]) {
					q.erase(make_pair(d[to], to));
					d[to] = d[v] + len;
					q.insert(make_pair(d[to], to));
				}
			}
		}

		if (d[f] == 10000000)
			cout << "-1";
		else
			cout << d[f];
	}