Cod sursa(job #996352)
#include<stdio.h>
#include<vector>
#include<queue>
#define pb push_back
#define mp make_pair
#define maxn 30005
#define maxb 8192
using namespace std;
int n,m,x,y;
int v[maxn],d[maxn];
vector < pair<int,int> > l[maxn];
char buff[maxb];
int pos=0;
int get_int()
{
int nr=0;
while(buff[pos]<'0' || buff[pos]>'9')
if(++pos==maxb) fread(buff,1,maxb,stdin),pos=0;
while(buff[pos]>='0' && buff[pos]<='9')
{
nr=nr*10+buff[pos]-'0';
if(++pos==maxb) fread(buff,1,maxb,stdin),pos=0;
}
return nr;
}
void read()
{
int a,b,c;
n=get_int(); m=get_int();
x=get_int(); y=get_int();
for(int i=1;i<=m;i++)
{
a=get_int(); b=get_int();
c=get_int();
l[a].pb(mp(b,c));
l[b].pb(mp(a,-c));
}
}
void bfs()
{
queue <int> q;
int k;
for(q.push(x),d[x]=0,v[x]=1;!q.empty();q.pop())
{
k=q.front();
for(unsigned int i=0;i<l[k].size();i++)
if(!v[l[k][i].first])
{
d[l[k][i].first]=d[k]+l[k][i].second;
v[l[k][i].first]=1;
q.push(l[k][i].first);
if(l[k][i].first==y) return;
}
}
}
int main()
{
freopen("sate.in","r",stdin);
freopen("sate.out","w",stdout);
read();
bfs();
printf("%d",d[y]);
fclose(stdin);
fclose(stdout);
return 0;
}