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

FILE *f=fopen("sate.in","r"), *g=fopen("sate.out","w");

int n,m,x,y;
//struct muchii{int s,d,c;}a[100025];
vector<pair <int,int> >v[30005];
int viz[30005],t[30005],c[30005];

int main(){
    int p,u,i,nod,j,s,d,co;
    fscanf(f,"%d%d%d%d",&n,&m,&x,&y);
    for(i=1;i<=m;i++){
        fscanf(f,"%d%d%d",&s,&d,&co);
        v[s].push_back(make_pair(d,co));
        v[d].push_back(make_pair(s,co));
    }
    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].first]==0){
                j=v[nod][i].first;
                u++;
                c[u]=j;
                //t[j]=nod;
                if(j>nod)
                    viz[j]=viz[nod]+v[nod][i].second;
                else
                    viz[j]=viz[nod]-v[nod][i].second;
                if(j==y)
                    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;
                }
        }
    }*/
    fprintf(g,"%d\n",viz[y]-1);
    return 0;
}