#include<cstdio>
#include<algorithm>
using namespace std;

long n,a[100001],b[100001];
long best[100001];
long i,g,tmax;

int main()
{
	freopen ("heavymetal.in","r",stdin);
	freopen ("heavymetal.out","w",stdout);
  
	scanf("%ld",&n);
  
	for(i=1;i<=n;i++)
	{
		scanf("%ld%ld",&a[i],&b[i]);
		if(b[i]>tmax)
			tmax=b[i];
	}
	
	for(i=1;i<=tmax;i++)
	{
		best[i]=best[i-1];
		for(long j=1;j<=n;j++)
			if(b[j]==i)
				best[i]=max(best[i],best[a[j]]+(b[j]-a[j]));
	}
  printf("%ld\n",best[tmax]);
  return 0;
}