PKU1860 Currency Exchange

#include<iostream>
#include<vector>
using namespace std;
const double eps=1e-8;
struct EDGE
{
    int f,e;
    double r,c;
}edge[300];

double d[300];

int main()
{
    int n,m,s;
    int i,j,k,t;
    double rab,cab,rba,cba;
    double v,_v,c,r;
    scanf("%d%d%d%lf",&n,&m,&s,&v);
    for(i=1;i<=n;++i)
        d[i]=-1e9;
    d[s]=v;
    for(k=0;k<m;++k)
    {
        scanf("%d%d%lf%lf%lf%lf",&i,&j,&rab,&cab,&rba,&cba);
        edge[2*k].f=i;
        edge[2*k].e=j;
        edge[2*k].c=cab;
        edge[2*k].r=rab;
        edge[2*k+1].f=j;
        edge[2*k+1].e=i;
        edge[2*k+1].c=cba;
        edge[2*k+1].r=rba;
    }
    while(1)
    {
        bool tag=true;
        for(t=0;t<2*m;++t)
        {
            i=edge[t].f;
            j=edge[t].e;
            c=edge[t].c;
            r=edge[t].r;
            _v=(d[i]-c)*r;
            if(_v>0.0&&_v>d[j]+eps)
            {
                d[j]=_v;
                tag=false;
            }
        }
        if(d[s]>v+eps||tag)break;
    }
    if(d[s]>v+eps)
        printf("YES\n");
    else
        printf("NO\n");
    return 0;
}