#include <fstream>
#include <vector>
using namespace std;
ifstream cin("sate.in");
ofstream cout("sate.out");
const int nmax=100024;
int dp[nmax];
bool viz[nmax];
int n,m,a,b,x,y,cost;
vector<pair<int,int> > v[nmax];
void dfs(int nod)
{
    for(auto e:v[nod])
        if(dp[e.first]!=-1)
    {
        dp[e.first]=dp[nod]+e.second;
        dfs(e.first);
    }
}
int main()
{
    cin>>n>>m>>a>>b;
    for(int i=1;i<=m;i++)
    {
        cin>>x>>y>>cost;
        v[x].push_back({y,cost});
        v[y].push_back({x,-cost});
    }
    for(int i=1;i<=n;i++)
        dp[i]=-1;
    dp[a]=0;
    dfs(a);
    cout<<dp[b]<<' ';
}