#include <fstream>
#include <vector>
#include <cstdio>
#include <queue>
#include <map>
#include <set>

#define NMAX 30005

using namespace std;

int n , m , st , en , x , y , cst;
int dp[NMAX];

bool c[NMAX];

vector <vector <pair <int , int> > > G;
queue <int> coada;

void bfs(int node);

void read(int &x) {
    int sign = 1;
    char ch;
    x = 0;

    while (!isdigit(ch = getchar())) {
        if (ch == '-') {
            sign = -1;
        }

        else
            sign = 1;
    }

    do {
        x = x * 10 + ch - '0';
    } while (isdigit(ch = getchar()));
    x *= sign;
}

int main() {
    freopen("sate.in" , "r" , stdin);
    freopen("sate.out"  ,"w" , stdout);

    read(n) , read(m) , read(st) , read(en);
    G.resize(n + 5);

    for (int i = 1; i <= m; ++i) {
        read(x) , read(y) , read(cst);

        G[x].push_back(make_pair(y , cst));
        G[y].push_back(make_pair(x , -cst));
    }

    bfs(st);
    printf("%d" , dp[en]);
    return 0;
}

void bfs(int node) {
    coada.push(node);
    c[node] = 1;

    while (!coada.empty()) {
        int node = coada.front();
        coada.pop();

        c[node] = 1;
        int nr = G[node].size();

        for (int i = 0; i < nr; ++i) {
            int it = G[node][i].first;
            int cost = G[node][i].second;

            if (c[it] == 0) {
                dp[it] = dp[node] + cost;
                coada.push(it);
            }
        }
    }
}