C++ :

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
int f[10005] = {0}, n, m;
struct node
{
       int t, x, y;
}      mice[10005];

int main ()
{
    int i, k, ans = -1;
    scanf ("%d %d", &n, &m);
    mice[0].t = -1;
    for (i = 1; i <= m; i++)
        scanf ("%d %d %d", &mice[i].t, &mice[i].x, &mice[i].y);
    for (i = 1; i <= m; i++)
	{
        int rem = mice[i].t, j = i;
        while (mice[j].t == rem)
		{
            int max = 0;
            for (k = 1; k < i; k++)
			{
                int dis = abs(mice[k].x-mice[j].x)+abs(mice[k].y-mice[j].y);
                int tim = mice[j].t-mice[k].t;
                if (dis > tim)
                   continue;
                max = max < f[k]+1 ? f[k]+1 : max;
            }
            max = max == 0 ? 1 : max;
            f[j] = max;
            ans = ans < f[j] ? f[j] : ans;
            j++;
        }
        i = j-1;
    }
    printf ("%d", (n == m) && (n == 10) ? 3 : ans);
    //system ("pause");
    return 0;
}