#include<iostream>
#include<queue>
using namespace std;
queue<int> u;
int x,y;
int wa,re;
int n;
int k[1000001],maxs;
int main(){
	cin>>n;
	wa=1;
	maxs=1;
	u.push(wa);
	for(int i=1;i<=n;i++){
		cin>>x>>y;
		wa=u.front();
		if(!u.empty()){
			u.pop();
		}
		if(x!=0){
			re=wa+1;
			if(maxs<re){
				maxs=re;
			}
			k[re]++;
			u.push(re);
		}
		if(y!=0){
			re=wa+1;
			if(maxs<re){
				maxs=re;
			}
			k[re]+=1;
			u.push(re);
		}
	}
	cout<<maxs;
	return 0;
}