#include <cstdio>
#include <vector>
#include <queue>
const int MAX=30111;
using namespace std;
struct sate{
    int varf,cost;
};
inline sate build(int a,int b){
    sate aux;
    aux.varf=a;
    aux.cost=b;
    return aux;
}
vector <sate> gr[MAX];
bool uz[MAX];
int sol[MAX];
void bfs(int start);
int main()
{
    int st,dr,n,m;
    freopen("sate.in","r",stdin);
    freopen("sate.out","w",stdout);
    scanf("%d%d%d%d",&n,&m,&st,&dr);
    for(int i=1;i<=m;++i){
        int sata,satb,cash;
        scanf("%d%d%d",&sata,&satb,&cash);
        gr[sata].push_back(build(satb,cash));
        gr[satb].push_back(build(sata,-cash));
    }
    bfs(st);
    printf("%d\n",sol[dr]);
    return 0;
}
void bfs(int start){
    queue <int> q;
    q.push(start);
    uz[start]=1;
    while(!q.empty()){
        int x=q.front();
        for(int i=0;i<(int)gr[x].size();++i)
            if(!uz[gr[x][i].varf]){
                uz[gr[x][i].varf]=1;
                sol[gr[x][i].varf]=sol[x]+gr[x][i].cost;
                q.push(gr[x][i].varf);
            }
        q.pop();
    }
}