#include <fstream>
#include <algorithm>
#define x first
#define y second
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
pair <int, int> v[50005];
int n,i,mij,d[100005],t[100005],w[100005],k2,k,p,u,x,sol,m;
int main()
{
    fin>>m>>n;
    for(i=1;i<=n;i++)
    {
        fin>>v[i].x>>v[i].y;
    }
    sort(v+1,v+n+1);
    d[1]=v[1].y;
    for(i=2;i<=n;i++)
    {
        sol=max(sol,d[i-1]+(v[i].x-v[i-1].x)+v[i].y);
        d[i]=max(d[i-1]+(v[i].x-v[i-1].x),v[i].y);
    }
    fout<<sol;
    return 0;
}