#include <fstream>
#include <vector>
#include <queue>

using namespace std;

class InputReader {
    public:
        InputReader() {}
        InputReader(const char *file_name) {
            input_file = fopen(file_name, "r");
            cursor = 0;
            fread(buffer, SIZE, 1, input_file);
        }
        inline InputReader &operator >>(int &n) {
            while(buffer[cursor] < '0' || buffer[cursor] > '9') {
                advance();
            }
            n = 0;
            while('0' <= buffer[cursor] && buffer[cursor] <= '9') {
                n = n * 10 + buffer[cursor] - '0';
                advance();
            }
            return *this;
        }
    private:
        FILE *input_file;
        static const int SIZE = 1 << 17;
        int cursor;
        char buffer[SIZE];
        inline void advance() {
            ++ cursor;
            if(cursor == SIZE) {
                cursor = 0;
                fread(buffer, SIZE, 1, input_file);
            }
        }
};

InputReader f ("sate.in");
ofstream t ("sate.out");

struct edges{
int dest,cost;};

vector <edges> v[30010];
int n,m,cost[30010];
bool vaz[30010];

int bfs(int nod,int target){int curent;
    queue <int> q;
    q.push(nod);
    while (!q.empty()){
    curent=q.front();
    vaz[curent]=true;
    q.pop();
        for (auto i:v[curent])
            if (!vaz[i.dest]){
            q.push(i.dest);
            if (i.dest>curent)
                cost[i.dest]=cost[curent]-i.cost;
            else
                cost[i.dest]=cost[curent]+i.cost;
            }
    }
    return 0;
}

int main()
{
    int x,y,t1,t2,c;
    f>>n>>m>>x>>y;
    for (int i=0;i<m;++i)
        f>>t1>>t2>>c,v[t1].push_back({t2,c}),v[t2].push_back({t1,c});
    if (x>y) swap (x,y);
    bfs(x,y);
    t<<cost[x]-cost[y];
    return 0;
}