#include<stdio.h>

#define NM 30001
#define MM 100025
struct dist{int a,b,d;};
dist v[MM+1],w[MM+1];
int pv[NM+1],pw[NM+1];
int n,m,x,y,dxy;
int c[NM+1],li=1,ls,viz[NM+1],d[NM+1];

int cvida(){return li>ls;}
void pune(int x){c[++ls]=x,viz[x]=1;}
int scoate(){return c[li++];}

void poz1(int st, int dr,int &piv){
int i=st,j=dr,d=0;
dist aux;
while(i<j){
	if(v[i].a>v[j].a||v[i].a==v[j].a&&v[i].b>v[j].b){
		aux=v[i],v[i]=v[j],v[j]=aux,d=1-d;
		}
	i+=d,j-=1-d;
	}
piv=i;
}
void qsrt1(int li,int ls){
int piv;
if(li<ls){
	poz1(li,ls,piv);
	qsrt1(li,piv-1);
	qsrt1(piv+1,ls);
	}
}
void poz2(int st, int dr,int &piv){
int i=st,j=dr,d=0;
dist aux;
while(i<j){
	if(w[i].b>w[j].b||w[i].b==w[j].b&&w[i].a>w[j].a){
		aux=w[i],w[i]=w[j],w[j]=aux,d=1-d;
		}
	i+=d,j-=1-d;
	}
piv=i;
}
void qsrt2(int li,int ls){
int piv;
if(li<ls){
	poz2(li,ls,piv);
	qsrt2(li,piv-1);
	qsrt2(piv+1,ls);
	}
}

int main(){
freopen("sate.in","r",stdin);
freopen("sate.out","w",stdout);
int i,p,gata=0,z,u;
scanf("%d%d%d%d",&n,&m,&x,&y);
for(i=1;i<=m;++i){
	scanf("%d%d%d",&v[i].a,&v[i].b,&v[i].d);
	w[i]=v[i];
	}
//qsrt1(1,m);
//qsrt2(1,m);
for(i=1;i<=m;++i){
	p=v[i].a;
	if(!pv[p]) pv[p]=i;
	}
for(i=1;i<=m;++i){
	p=w[i].b;
	if(!pw[p]) pw[p]=i;
	}
pune(x);/*
while(!gata&&!cvida()){
	z=scoate();
	i=pv[z];
	if(i){
		while(i<=m&&v[i].a==z){
			u=v[i].b;
			if(!viz[u]) {
				pune(u);
				if(u>z) d[u]=d[z]+v[i].d;
				else d[u]=d[z]-v[i].d;
				}
			if(u==y) {dxy=d[u];gata=1;break;}
			i++;
			}
		}
	if(gata) break;
	i=pw[z];
	if(i){
		while(i<=m&&w[i].b==z){
			u=w[i].a;
			if(!viz[u]) {
				pune(u);
				if(u>z) d[u]=d[z]+w[i].d;
				else d[u]=d[z]-w[i].d;
				}
			if(u==y) {dxy=d[u];gata=1;break;}
			i++;
			}
		}
	}  */
printf("%d",dxy);
return 0;
}