import java.io.BufferedReader;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.StringTokenizer;
import java.util.ArrayList;

public class cMain implements Runnable {
	StringTokenizer st;
	BufferedReader in;
	PrintWriter out;

	ArrayList<Integer>[] graph;
	ArrayList<Integer>[] dstgraph;

	int color[];

	int n;
	int m;
	int time;
	int pr[];
	int counter;
	int answer[];
	int d[];
	int flag;
	int start, finish;

	public static void main(String[] args) {
		new Thread(new cMain()).run();
	}

	public void run() {
		try {
			in = new BufferedReader(new FileReader("shortpath.in"));
			out = new PrintWriter(new FileWriter("shortpath.out"));
			solve();
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			out.flush();
			out.close();
		}
	}

	String nextToken() throws IOException {
		while (st == null || !st.hasMoreTokens()) {
			st = new StringTokenizer(in.readLine());
		}
		return st.nextToken();
	}

	int nextInt() throws NumberFormatException, IOException {
		return Integer.parseInt(nextToken());
	}

	long nextLong() throws NumberFormatException, IOException {
		return Long.parseLong(nextToken());
	}

	double nextDouble() throws NumberFormatException, IOException {
		return Double.parseDouble(nextToken());
	}

	void DFSVISIT(int a) {
		color[a] = 1;
		int s = graph[a].size();
		for (int i = 0; i < s; i++) {
			int ii = graph[a].get(i);
			if (color[ii] == 1) {
				flag = 1;
			}
			if (color[ii] == 0) {

				DFSVISIT(ii);
			}

		}
		color[a] = 2;

		answer[counter] = a;
		counter++;
	}

	void DFS() {
		DFSVISIT(start);
		/*for (int i = 0; i < n; i++) {
			if (color[i] == 0) {
				DFSVISIT(i);
				if (flag == 1) {
					return;
				}
			}
		}*/
	}

	void ISS(int a) {
		for (int i = 0; i < n; i++) {
			d[i] = 1000000;
		}
		d[a] = 0;
	}

	void relax(int u, int v, int i) {
		int w = dstgraph[u].get(i);
		if (d[v] > d[u] + w) {
			d[v] = d[u] + w;
			pr[v] = u;
		}
	}

	@SuppressWarnings("unchecked")
	void solve() throws NumberFormatException, IOException {
		n = nextInt();
		m = nextInt();
		start = nextInt();
		finish = nextInt();
		start--;
		finish--;

		d = new int[n];
		color = new int[n];
		pr = new int[n];
		answer = new int[n];
		graph = new ArrayList[n];
		dstgraph = new ArrayList[n];
		for (int i = 0; i < n; i++) {
			graph[i] = new ArrayList<Integer>();
			dstgraph[i] = new ArrayList<Integer>();
		}
		for (int i = 0; i < m; i++) {
			int a, b, w;

			a = nextInt();
			b = nextInt();
			w = nextInt();
			graph[a - 1].add(b - 1);
			dstgraph[a - 1].add(w);

		}

		DFS();
		ISS(start);

		for (int i = n - 1; i != -1; i--) {
			int s = graph[answer[i]].size();

			for (int ii = 0; ii < s; ii++) {

				relax(answer[i], graph[answer[i]].get(ii), ii);

			}

		}
		if (d[finish] != 1000000) {
			out.print(d[finish]);
		} else {
			out.print("Unreachable");
		}

	}
}