#pragma comment (linker, "/STACK:64000000")
#include <fstream>
#include <vector>
#include <stack>
#include <cstdio>

using namespace std;

struct edje_to
{
    int to;
    int weght;
};

vector<edje_to>* edjes;
vector<int> nodes;
int* colors;
int* weights;

int m, n;
int a, b;
int s, t;
edje_to p;
bool path = false;

void topsort(int v)
{
    colors[v] = 1;
    for (int i = 0; i < edjes[v].size(); i++)
    {
        if (colors[edjes[v][i].to] == 0)
        {
            topsort(edjes[v][i].to);
        }
    }
    colors[v] = 2;
    nodes.push_back(v);
}

int main()
{
    ifstream in;
    ofstream out;

    in.open("shortpath.in");
        in >> n >> m >> s >> t;
        edjes = new vector<edje_to> [n];
        colors = new int [n];
        weights = new int [n];
        for (int i = 0; i < n; i++)
        {
            colors[i] = 0;
            weights[i] = 2100000000;
        }
        for (int i = 0; i < m; i++)
        {
            in >> a >> p.to >> p.weght;
            p.to--;
            edjes[a - 1].push_back(p);
        }
    in.close();

    for(int i = 0; i < n; i++)
    {
        if(colors[i] == 0)
        {
            topsort(i);
        }
    }

    a = 0;

    for(int i = nodes.size() - 1; i > 0; i--)
    {
        if(nodes[i] == s - 1)
        {
            a = i;
            break;
        }
    }

    weights[s - 1] = 0;
    for(int i = a; i >= 0; i--)
    {
        for(int j = 0; j < edjes[nodes[i]].size(); j++)
        {
            if(weights[edjes[nodes[i]][j].to] > weights[nodes[i]] + edjes[nodes[i]][j].weght)
            {
                weights[edjes[nodes[i]][j].to] = weights[nodes[i]] + edjes[nodes[i]][j].weght;
            }
        }
    }

    out.open("shortpath.out");
        if(weights[t - 1] == 2100000000)
        {
            out << "Unreachable";
        }
        else
        {
            out << weights[t - 1];
        }
    out.close();

    return 0;
}