#include <bits/stdtr1c++.h>
#define maxn 30010
using namespace std;

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

int Cost[maxn], V[maxn], S[maxn];
vector <int> A[maxn];
vector <int> D[maxn];
int N, M, X, Y;

void init() {
    fin >> N >> M >> X >> Y;
    int x, y ,c;
    for(int i=0; i < M; i++) {
        fin >> x >> y >> c;
        A[x].push_back(y);
        D[x].push_back(c);
        A[y].push_back(x);
        D[y].push_back(-c);
    }
    Cost[X] = 0;
}

void BFS(int nod) {
    memset(V, -1, sizeof(V));
    int L=1;
    S[L] = nod;
    V[nod] = 1;
    for(int i=1; i<=L;i++) {
        for(int j=0; j < A[S[i]].size(); j++) {
            if(V[A[S[i]][j]] == -1) {
                S[++L] = A[S[i]][j];
                V[A[S[i]][j]] = 1;
                Cost[A[S[i]][j]] = Cost[S[i]] + D[S[i]][j];
            }
        }
    }
}

int main()
{
    init();
    BFS(X);
    fout << Cost[Y];
    return 0;
}