#include<iostream>
#include<algorithm>
#include<cstdio>
using namespace std;
int main()
{
	int n = 1;//每一层的非叶子节点 
	int dep = 1;//深度 
	int num = 0;
	cin>>num;
	int next = 0;//下一层的非叶子节点 
	while(n--)
	{
		int a = 0, b = 0;
		cin>>a>>b;
		if((a != 0&& b == 0)||(a == 0&&b != 0))
		{
			next++;
		}
		else if(a != 0 && b != 0)
		{
			next += 2;
		}
		if(n == 0)//这一层的子节点输入完之后 
		{ 
			n = next;
			next = 0;
			dep++;
		}
	}
	cout<<dep<<endl;
}