#include <bits/stdc++.h>
using namespace std;
struct snz
{
	long long num,time;
};
int  main()
{
	long long n;
	cin>>n;
	snz a[n+1];
	for(long long i=1;i<=n;i++)
	{
		cin>>a[i].time>>a[i].num;
	}
	long long sum=0,o=1;
	for(long long i=1;i<=a[n].time;i++)
	{
		if(a[o].time==i)
		{
			sum+=a[o++].num;
			//cout<<"sum="<<sum<<" ";
			if(o==n)
			{
				continue;
			}
			else
			{
				sum--;
			}
			continue;
		}
		if(sum!=0)
		{
			sum--;
		}
	//	cout<<sum<<" "<<o<<"  ";
	}
	cout<<sum; 
	return 0;
}