#include <bits/stdc++.h>
#define PI 2*asin(1)
#define FOR(i,n) for(long long i = 0; i < n;i++)
using namespace std;
long long n,m,s,t;
vector <long long>res;
vector<vector<pair<long long,long long>>>rast(100001);
void go(long long v){
    for(long long i = 0; i < rast[v].size(); i++){
        res[rast[v][i].first] = min(res[rast[v][i].first], res[v]+rast[v][i].second);
        go(rast[v][i].first);
    }
}
int main()
{
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);

    cin >> n >> m >> s >> t; s--;t--;
    FOR(i,n) res.push_back(INT_MAX);
    res[s] = 0;
    FOR(i,m){
        long long x,y,z;cin >> x >> y >> z; x--;y--;
        rast[x].push_back({y,z});
    }
    go(s);
    long long y = res[t];
    if(y<INT_MAX)cout << res[t];
    else cout << "Unreachable";
}