#include <fstream>
#include <algorithm>

using namespace std;

ifstream ci ("orase.in");
ofstream co ("orase.out");

struct str
{
    int x,y;
};

const int nmax=50000;
str v[nmax+1];

bool cmp (str a,str b)
{
    return a.x<b.x;
}

int main()
{
    int m,n;
    ci >> m >> n;
    for (int i=1; i<=n; i++)
    {
        ci >> v[i].x;
        ci >> v[i].y;
    }
    sort(v+1, v+n+1, cmp);
    int max=v[1].y;
    int sol=0;
    for (int i=2; i<=n; i++)
    {
        max+=v[i].x-v[i-1].x;
        if (v[i].y+max>sol)
        {
            sol=v[i].y+max;
        }
        if (v[i].y>max)
        {
            max=v[i].y;
        }
    }
    co << sol;
    return 0;
}