#include <iostream>
#include<ctime>
#include<map>
#include<vector>
#include<queue>
using namespace std;
vector < pair<int,int> > g[10005];
bool visited[10005];
int main()
{
    int t1,t2;
    cin>>t1>>t2;
    int k,n;
    cin>>k>>n;

    for(int i=0;i<n;i++)
    {
        int a,b,c;
        cin>>a>>b>>c;
        g[a].push_back(make_pair(b,c));
        g[b].push_back(make_pair(a,c));
    }
    queue <long long> q;
    q.push(t1);
    q.push(0);

    for(int i=0;i<=k;i++)
    {
        visited[i]=false;
    }
    while(!q.empty())
    {
        long long node=q.front();
        q.pop();
        long long dis=q.front();
        q.pop();
        if(node==t2)
        {
            cout<<llabs(dis);
            return 0;
        }
        for(int i=0;i<g[node].size();i++)
        {
            int sosed=g[node][i].first;
            long long d=g[node][i].second;
            if(!visited[sosed])
            {
               visited[sosed]=true;
               q.push(sosed);
               if(node<sosed)
               {
                   q.push(dis+d);
               }else
               {
                   q.push(dis-d);
               }
            }

        }
    }
    cout<<-1;
    return 0;
}