#include<fstream>
#include<algorithm>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
int n,i,m,max1,best;
struct pie
{
    int d,l;
}s[50001];
int cmp(pie a,pie b)
{
    if(a.d==b.d)
        return a.l<b.l;
    return a.d<b.d;
}
int main ()
{
    f>>m>>n;
    for(i=1;i<=n;++i)
        f>>s[i].d>>s[i].l;
    sort(s+1,s+n+1,cmp);
    for(i=1;i<=n;++i)
    {
        max1+=s[i].d-s[i-1].d;
        max1=max(s[i-1].l+s[i].d-s[i-1].d,max1);
        if(max1+s[i].l>best)
            best=max1+s[i].l;
    }
    g<<best;
    return 0;
}