#include <fstream>
#include <algorithm>
using namespace std;
struct strada{int d, l;
             }v[50005];
int n, i, j, ma[50005], m, maxi;
ifstream f("orase.in");
ofstream g("orase.out");
bool mycmp(strada i, strada j)
{
    return i.d<j.d;
}
int main()
{
    f>>m>>n;
    for(i=1;i<=n;++i) f>>v[i].d>>v[i].l;
    sort(v+1,v+n+1,mycmp);
    for(i=1;i<=n;++i) ma[i]=max(ma[i-1],v[i].l-v[i].d);
    for(i=1;i<=n;++i) maxi=max(maxi,v[i].l+v[i].d+ma[i-1]);
    g<<maxi<<"\n";
    return 0;
}