#include <cstdio>
#include <algorithm>
#include<vector>
#include<queue>

using namespace std;

int d[30009],y,nod,minim,i,aux,y1,n,k,x2,x1,j,p,s,unu,t,m,doi,x,st,dr,maxi,sol,maxi2,ok,cost;
vector < pair<int,int> > v[30010];
int viz[30009];

queue<int > q;

void bfs(int start)
{
    d[start]=0;
    q.push(start);
    while(! q.empty())
    {
        nod=q.front();
        //printf("%d ",nod);
        q.pop();
        for(i=0;i<v[nod].size();i++)
        {
            //printf("%d %d\n",nod,v[nod][i].first);
            if(d[v[nod][i].first]==-1)
            {
               // printf("%d ",v[nod][i].second);
                d[v[nod][i].first]=v[nod][i].second+d[nod];
                q.push(v[nod][i].first);
            }

        }
    }
}

int main()
{
    freopen ("sate.in","r",stdin);
    freopen ("sate.out","w",stdout);
    scanf("%d%d%d%d",&n,&m,&x1,&x2);
    for(i=1;i<=m;i++)
    {
        scanf("%d %d %d",&x,&y,&cost);
       // printf("%d %d\n",x,y);
        v[x].push_back(make_pair(y,cost));
        v[y].push_back(make_pair(x,-cost));
    }
    for(i=1;i<=n;i++) d[i]=-1;
    bfs(x1);
    for(i=1;i<=n;i++)
    {
        //printf("%d ",d[i]);
    }
    printf("%d\n",d[x2]);
}