#include <fstream>
#include <cstdlib>
#define mx LONG_LONG_MAX
#define nmax 30050
#include <queue>
#include <vector>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");
int n,m,x,y,i,a,b,nod;
long long dist,dis,di,d[nmax];
vector <pair <int,long long> >v[nmax];
queue <pair<int,long long> > q;
bool ok,use[nmax];
int main()
{
f>>n>>m>>x>>y;
f.sync_with_stdio(false);
for(i=1; i<=m; ++i)
{
f>>a>>b>>dis;
v[a].push_back({b,dis});
v[b].push_back({a,-dis});
}
q.push({x,0});
while(!q.empty())
{
nod=q.front().first;
di=q.front().second;
q.pop();
for(i=0; i<v[nod].size(); ++i)
{
if(use[v[nod][i].first]==0)
{
use[v[nod][i].first]=1;
d[v[nod][i].first]=di+v[nod][i].second;
q.push({v[nod][i].first,d[v[nod][i].first]});
}
if(d[y]!=0) g<<d[y],exit(0);
}
}
return 0;
}