#include<iostream>
using namespace std;
int z[10][20005];
int main(){
	int n=0,time=0,ans=0,p,t,c=0,situation=0;
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>p>>t;
		z[p][z[p][0]+1]=t;
		z[p][0]++;
		c++;
	}
	for(int i=1;i<=6;i++){
		for(int j=1;j<=z[i][0];j++){
			if(time>=59){
				time=0;
				ans+=1;
			}
			time+=z[i][j]-z[i][j-1];
			if(j==z[i][0]&&time!=0){
				ans++;
			} 
		}
	}
	cout<<ans;
}