#include<stdio.h>
#define N 30002
int n,m,x,y;
int *a[N],*b[N],deg[N]={0},v[N];

struct muchie{
	int x,y,z;
}w[N];

void citire(){
	int i;
	scanf("%d%d%d%d",&n,&m,&x,&y);
	for(i=1;i<=m;++i){
		scanf("%d%d%d",&w[i].x,&w[i].y,&w[i].z);
		deg[w[i].x]++;
		deg[w[i].y]++;
	}
	for(i=1;i<=n;++i){
		a[i]=new int[deg[i]+1];
		a[i][0]=0;
		b[i]=new int[deg[i]+1];
		b[i][0]=0;
	}
	for(i=1;i<=m;++i){
		a[w[i].x][++a[w[i].x][0]]=w[i].y;
		a[w[i].y][++a[w[i].y][0]]=w[i].x;
		b[w[i].x][++b[w[i].x][0]]=w[i].z;
		b[w[i].y][++b[w[i].y][0]]=-w[i].z;
	}
}

void bfs(){
	int u=0,p=1,i,x1,coada[N];
	bool viz[N]={false};
	coada[++u]=x;
	while(u>=p){
		x1=coada[p++];
		for(i=1;i<=a[x1][0];++i)
			if(!viz[a[x1][i]]){
				coada[++u]=a[x1][i];
				viz[a[x1][i]]=true;
				v[a[x1][i]]=b[x1][i]+v[x1];
			}
	}
	printf("%d\n",v[y]);
}

int main(){
	freopen("sate.in","r",stdin);
	freopen("sate.out","w",stdout);
	citire();
	bfs();
	return 0;
}