#include<fstream>
#include<vector>
#include<queue>
#include<cstring>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");
int a,b,c,n,m,x,y,i,d[30010];
struct nod{int x,cost;};
nod v;
vector<nod>l[30010];
queue<int>q;
int main()
{
    f>>n>>m>>x>>y;
    for(i=1;i<=m;++i)
    {
        f>>a>>b>>c;
        v.x=b;
        v.cost=c;
        l[a].push_back(v);
        v.x=a;
        v.cost=-c;
        l[b].push_back(v);
    }
    memset(d,-1,sizeof(d));
    d[x]=0;
    q.push(x);
    while(!q.empty())
    {
        x=q.front();
        for(vector<nod>::iterator it=l[x].begin();it!=l[x].end();++it)
        {
            v=*it;
            if(d[v.x]==-1)
            {
                d[v.x]=d[x]+v.cost;
                q.push(v.x);
            }
        }
        q.pop();
    }
    g<<d[y]<<'\n';
    return 0;
}