#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
#include <math.h>

using namespace std;

ifstream f("sate.in");
ofstream g("sate.out");

vector<bool> viz;
queue<int> q;

struct muchie{
    int dr,d,urm;
}M[200050];

int cap[30010],ctr=0;
long long poz[30010];

void adauga(int a,int b,int d) {
    ctr++;
    M[ctr].dr = b;
    M[ctr].d = d;
    M[ctr].urm = cap[a];
    cap[a] = ctr;
}

int main() {
    int n,m,x,y;
    f>>n>>m>>x>>y;
    viz.assign(n+1,false);
    
    for(int i=1;i<=m;i++) {
        int st,dr,D;
        f>>st>>dr>>D;
        adauga(st,dr,D);
        adauga(dr,st,-D);
    }
    poz[x] = 0;
    q.push(x);
    while(!q.empty()) {
        int nod = q.front();
        q.pop();
        int i = cap[nod];
        while(i!=0) {
            if(viz[M[i].dr]==false) {
                viz[M[i].dr]=true;
                poz[M[i].dr]=poz[nod]+M[i].d;
                q.push(M[i].dr);
            }
            i = M[i].urm;
        }
    }
    g<<abs(poz[y]-poz[x]);


    return 0;
}