#include<fstream>
#include<algorithm>

using namespace std;

const char iname[]="orase.in";
const char oname[]="orase.out";
const int maxn=100005;

ifstream f(iname);
ofstream g(oname);

pair<int,int> a[maxn];

int i,n,maxt,mint=-0x3f3f3f3f;

int main()
{
    f>>n>>n;
    for(i=1;i<=n;++i)
        f>>a[i].first>>a[i].second;
    sort(a+1,a+n+1);
    for(i=1;i<=n;++i)
    {
        mint+=a[i].first-a[i-1].first;
        maxt=max(maxt,a[i].second+mint);
        mint=max(mint,a[i].second);
    }
    g<<maxt<<"\n";
}