#include<stdio.h>
#include<vector>
using namespace std;
#define nmax 30005
struct element{long n, c;};
long n, m, x, y, a, b, c, i;
element el;
vector <element> ma[nmax];
bool viz[nmax];
long dist[nmax];

void citire()
{
    scanf("%ld %ld %ld %ld",&n,&m,&x,&y);
    for (i=1;i<=m;i++)
    {
        scanf("%ld %ld %ld",&a,&b,&c);
        el.n=b; el.c=c; ma[a].push_back(el);
        el.n=a; el.c=-c; ma[b].push_back(el);
    }
}

void dfs(long x)
{
    viz[x]=1;
    vector <element> ::iterator it;
    for (it=ma[x].begin();it!=ma[x].end();it++)
        if (!viz[(*it).n])
        {
            dist[(*it).n]=dist[x]+(*it).c;
            dfs((*it).n);
        }
}

int main()
{
    freopen("sate.in","r",stdin);
    freopen("sate.out","w",stdout);
    citire();
    dfs(x);
    printf("%ld",dist[y]);
    return 0;
}