#include <bits/stdc++.h>
using namespace std;
ifstream f("heavymetal.in");
ofstream g("heavymetal.out");
int x[100001],i,j,nr,n,mx;
struct hm
{
    int x,y;
}v[100001];
int comp(hm a, hm b)
{
    return a.y<b.y;
}
int main()
{
    f>>n;
    for (i=1;i<=n;i++)
        f>>v[i].x>>v[i].y;
    sort(v+1,v+n+1,comp);
    mx=v[n].y;
    for (i=1;i<=mx;i++)
    {
        x[i]=x[i-1];
        for (j=1;j<=n;j++)
            if(v[j].y==i) x[i]=max(x[i],x[v[j].x]+(v[j].y-v[j].x));
    }
    g<<x[mx];
    return 0;
}