//#include<algorithm>
//using namespace std;
#include<stdio.h>

#define DIM 30001

int n,m,in,sf,c[DIM],val[DIM];

struct nod{
    int x,d;
    nod *urm;};
nod *lst[DIM];

void add(int a,int b,int d){
    nod *p=new nod;

    p->x=b;
    p->d=d;
    p->urm=lst[a];
    lst[a]=p;}

void bf(){
    int st,dr;
    nod *p;

    for(c[st=dr=0]=in; st<=dr; ++st)
        for(p=lst[c[st]]; p; p=p->urm)
            if(p->x&&!val[p->x]){
                if(p->x<c[st])
                    val[p->x]=val[c[st]]-p->d;
                else
                    val[p->x]=val[c[st]]+p->d;
                c[++dr]=p->x;}}

void solve(){
    int i,a,b,d;

    scanf("%d%d%d%d",&n,&m,&in,&sf);
    for(i=0; i<m; ++i){
        scanf("%d%d%d",&a,&b,&d);
        add(a,b,d);
        add(b,a,d);}
    bf();
    printf("%d",val[sf]);}

int main(){

	freopen("sate.in","r",stdin);
    freopen("sate.out","w",stdout);

    solve();

    return 0;}