#include<iostream>
#define maxn 10000
using namespace std;
int a[maxn];
int main()
{
	int n;
	cin>>n;
	int height=1;
	int index=2;
	int temp=0;
	while(true)
	{
		int temp=0;
		for(int i=1;i<=index;i++)
		{
			cin>>a[i];
			if(a[i]!=0)
				temp++;
		}
		index=temp*2;
		height+=1;
		if(index==0)
			break;
	}
	cout<<height;
}