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

typedef vector<int> VI;
typedef VI::iterator IT;

ifstream fin("sate.in");
ofstream fout("sate.out");

int n, m, A, B, km;
VI g[30001];
VI d;
queue<int> q;
vector<bool> s;

void Read();
void BFS( int nod );

int main()
{
    Read();
    BFS( A );
    fin.close();
    fout.close();
    return 0;
}


void Read()
{
    int x, y, c;
    fin >> n >> m >> A >> B;
    d.resize(n + 1);
    s.resize(n + 1);
    for( int i = 0; i < m; ++i )
    {
        fin >> x >> y >> c;
        g[x].push_back( y );
        g[y].push_back( x );
        d[x] = c;
        d[y] = -c;
    }
}
void BFS( int nod )
{
    int i;
    km = d[nod];
    s[nod] = true;
    q.push( nod );
    while( !q.empty() )
    {
        i = q.front();
        q.pop();
        for( IT it = g[i].begin(); it != g[i].end(); ++it )
            if( !s[*it] )
            {
                s[*it] = true;
                q.push( *it );
                km += d[*it];
                if( *it == B )
                {
                    fout << km << '\n';
                    return;
                }

            }
    }
}