#include <fstream>
#include <algorithm>

using namespace std;
ifstream cin("orase.in");
ofstream cout("orase.out");
pair <int,int> v[50005];
int m,n,i,lg,d[50005],di;
int main()
{
    cin>>m>>n;
    for(i=1;i<=n;i++)
        cin>>v[i].first>>v[i].second;
    sort(v+1,v+n+1);
    di=0;
    d[1]=v[1].second;
    for(i=1;i<=n;i++)
    {
        if(d[i-1]+v[i].first-v[i-1].first+v[i].second>lg)
            lg=d[i-1]+v[i].first-v[i-1].first+v[i].second;
        d[i]=max(v[i].second,d[i-1]+v[i].first-v[i-1].first);
    }
    cout<<lg;
    return 0;
}