#include <fstream>
#include <cstdio>
#include <vector>
using namespace std;

vector<int> tsort;

int n, m;
int s,t;
int v1, v2, v3;
vector<pair<int, int>> *arr;
bool* visited;


void dfs(int i)
{
    visited[i] = true;
    for(size_t j = 0; j < arr[i].size(); j++)
        if (!visited[arr[i][j].first])
            dfs(arr[i][j].first);

    tsort.push_back(i);
}

int main()
{
    ifstream in("shortpath.in");
    ofstream out("shortpath.out");

    in >> n >> m >> s >> t;

    --s;
    --t;
    visited = new bool [n];
    arr = new vector<pair<int,int>> [n];
    for (int i = 0; i < n; i++)
    {
        visited[i] = false;;
    }

    pair<int, int> p;
    for(int i = 0; i < m; i++)
    {
        in >> v1 >> v2 >> v3;
        v1--;
        v2--;
        p.first = v2;
        p.second = v3;
        arr[v1].push_back(p);
    }

    for(int i = 0; i < n; i++)
        if(!visited[i])
            dfs(i);


    int v;
    int *d;
    d = new int [n];
    for (int i = 0; i < n; i++)
        d[i] = 1001;
    d[s] = 0;
    for(int i = (n - 1); i >= 0 ; i--)
    {
        v = tsort[i];
        if (d[v] == 1001) continue;

        for (size_t j = 0; j < arr[v].size(); j++)
            d[arr[v][j].first] = min(d[arr[v][j].first],d[v] + arr[v][j].second);

    }

    if(d[t] == 1001)
        out << "Unreachable";
    else
        out << d[t];
}