#include <fstream>
#include<vector>
using namespace std;

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

int n,m,x,y;
struct muchii{int s,d,c;}a[100025];
vector<int>v[30001];
int viz[30001],t[30001],c[30001];

int main(){
    int p,u,i,nod,j;
    f>>n>>m>>x>>y;
    for(i=1;i<=m;i++){
        f>>a[i].s>>a[i].d>>a[i].c;
        v[a[i].s].push_back(a[i].d);
        v[a[i].d].push_back(a[i].s);
    }
    p=u=1;
    c[p]=x;
    viz[x]=1;
    t[x]=0;
    while(p<=u){
        nod=c[p];
        for( i=0; i<v[nod].size() && p<=u; i++)
            if(viz[v[nod][i]]==0){
                u++;
                c[u]=v[nod][i];
                t[v[nod][i]]=nod;
                viz[v[nod][i]]=1;
                if(viz[y]==1)
                    p=u+1;
            }
        p++;
    }
    p=0;
    for(i=y;i>0;i=t[i]){
        u=t[i];
        for(j=1;j<=m;j++){
            if(a[j].s==u && a[j].d==i){
                p=p+a[j].c;
                j=m+1;
                }
            else
                if(a[j].s==i && a[j].d==u){
                    p=p-a[j].c;
                    j=m+1;
                }
        }
    }
    g<<p<<"\n";
    return 0;
}