#include <iostream>
#include <set>
#include <queue>
#include <vector>
using namespace std;
const int maxn = 1e5 + 5;
typedef long long ll;
int n, m;
vector<pair<int, int>> graph[maxn];
vector<bool> visited(maxn, false);

ll bfs(int S, int E){
    queue<pair<int, ll>> q;
    q.push({S, 0});

    visited[S] = true;

    while(!q.empty()){
        int node = q.front().first;
        ll total = q.front().second;
        q.pop();

        if(node == E){
            return llabs(total);
        }

        for(pair<int,int> edge : graph[node]){
            int neighbour = edge.first;
            int w = edge.second;

            if(!visited[neighbour]){
                visited[neighbour] = true;

                ll nov_total = total;
                if(neighbour > node){               //poradi tezinata, odam so ovoj nacin, ne so nova varijabla koja ja stavam vo queue
                    nov_total += w;
                }
                else{
                    nov_total -= w;
                }

                q.push({neighbour, nov_total});
            }
        }
    }

    return -1;
}

int main() {
    ios::sync_with_stdio(false);

    int t1, t2;
    cin >> t1 >> t2;


    cin >> n >> m;

     for(int i = 0; i < m; i++){
        int a, b, c;
        cin >> a >> b >> c;

        graph[a].push_back({b, c});
        graph[b].push_back({a, c});
    }

    cout << bfs(t1, t2);


    return 0;
}