//Earl_WR 2017.1.19
#include <iostream>
#include <cstdio>
#include <string>
#include <cstring>
#include <algorithm>
using namespace std;
int result=0;
int maxx=-1;
int n,k;
int a[10010],t_f,t_t,t_e;
int read()
{int x=0,f=1;char ch=getchar();while (ch<'0' || ch>'9'){if (ch=='-')f=-1;ch=getchar();}while (ch>='0' && ch<='9'){x=x*10+ch-'0';ch=getchar();}return x*f;
}
void in()
{n=read();k=read();for (int i=1;i<=k;i++){a[i]=read();}
}
int main()
{//freopen ("test.in","r",stdin);//freopen ("test.out","w",stdout);in();sort(a+1,a+1+k);t_f=n-a[k];if (t_f>result)result=t_f;t_t=a[1];if (t_t>result)result=t_t;for (int i=1;i<=k;i++){if (a[i+1]-a[i]>maxx){maxx=a[i+1]-a[i];}}t_e=maxx/2;if (t_e>result)result=t_e;cout<<result<<endl;return 0;
}

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <ctime>
#include <iomanip>
#include <algorithm>
#include <cstdlib>
#include <string>
using namespace std;
#define M 100000
struct T
{int v,t;
}q[M];
int head=1,tail=0,id[M],ans=0,now[M];
int map[1000][1000],m,n;
void init()
{memset(map,0,sizeof(map));memset(id,0,sizeof(id));cin>>n>>m;for(int i=1;i<=m;i++){int x,y,v;cin>>x>>y>>v;map[x][y]=v;id[y]++;}
}
void work()
{for(int i=0;i<n;i++)if(id[i]==0){q[++tail].v=i;q[tail].t=1;}for(head=1;head<=tail;head++){for(int i=0;i<n;i++)if(map[q[head].v][i]){id[i]--;now[i]=max(now[i],q[head].t+map[q[head].v][i]);if(id[i]==0){q[++tail].v=i;q[tail].t=now[i];}}}
}
int main()
{//freopen("1828.in","r",stdin);//freopen("1828.out","w",stdout);
    init();work();for(int i=1;i<=tail;i++){//cout<<i<<' '<<q[i].v<<' '<<q[i].t<<endl;ans=max(ans,q[i].t);}cout<<ans<<endl;return 0;
}