import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;


@SuppressWarnings("ALL")
public class Main {
    Edge[] v;
    long[] dist;
    int n;

    public static void main(String[] args) throws IOException {
        new Main().run();
    }

    private void run() throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        //Arrays.stream(reader.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
        int[] nmst = Arrays.stream(reader.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
        n = nmst[0];
        int m = nmst[1];
        nmst[2]--;
        nmst[3]--;
        v = new Edge[n];
        dist = new long[n];
        for (int i = 0; i < nmst[1]; i++) {
            int[] q = Arrays.stream(reader.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
            v[i] = new Edge(q[0] - 1, q[1] - 1, q[2]);
        }
        for (int i = 0; i < n; i++) {
            dist[i] = Long.MAX_VALUE;
        }
        dist[nmst[2]] = 0;

        for (int i = 0; i < n - 1; i++) {
            for (Edge current : v) {
                dist[current.b] = Long.min(dist[current.b], dist[current.a] + current.w);
            }
        }
        if (dist[nmst[3]] == Long.MAX_VALUE)
            System.out.println("Unreachable");
        else
            System.out.println(dist[nmst[3]]);
    }

    private class Edge {
        int a;
        int b;
        int w;

        public Edge(int a, int b, int w) {
            this.a = a;
            this.b = b;
            this.w = w;
        }
    }

}