#include <bits/stdc++.h>

using namespace std;
const int NMAX=10005;
int n,i,x,y,Max,dp[NMAX];
vector <int> v[NMAX];
int main()
{
    freopen("heavymetal.in","r",stdin);
    freopen("heavymetal.out","w",stdout);
scanf("%d /n", &n);
for (int i=1; i <= n; ++i)
{scanf("%d %d \n", &x, &y);
    v[y].push_back(x);
    Max=max(Max,y);

}
dp[0]=0;
for (i = 1; i <= Max; ++i)
{dp[i]=dp[i-1];
for (auto &it: v[i])
dp[i]=max(dp[i],dp[it]+i-it);

}
printf("%d \n", dp[Max]);

    return 0;
}