#include <cstdio>
#include <vector>

using namespace std;

int vaz[30010];
int d[30010];

struct edge
{
    int nod,s;
    edge(int nod1,int s1) {nod=nod1;s=s1;}
};

vector<edge> v[30010];

void dfs(int nod)
{
    vaz[nod]=1;
    for(int i=0;i<v[nod].size();i++)
        if(vaz[v[nod][i].nod]==0)
        {
            d[v[nod][i].nod]=v[nod][i].s+d[nod];
            dfs(v[nod][i].nod);
        }
}

int main()
{
    freopen("sate.in","r",stdin);
    freopen("sate.out","w",stdout);
    int n,m,x,y,a,b,s;
    scanf("%d%d%d%d",&n,&m,&x,&y);
    for(int i=1;i<=m;i++)
    {
        scanf("%d%d%d",&a,&b,&s);
        v[a].push_back(edge(b,s));
        v[b].push_back(edge(a,-s));
    }
    dfs(x);
    printf("%d",d[y]);
    return 0;
}