#include <fstream>
#include <vector>

using namespace std;

ifstream fin("orase.in");
ofstream fout("orase.out");

int main()
{
    int m, n;
    fin>>m>>n;

    vector<int> ds(n + 1);
    vector<int> d(n + 1);

    int dmax = (1 << 32), ddif, dsuma;
    for(int i = 1; i <= n; i++)
    {
        fin>>ds[i]>>d[i];
    }
    for(int i = 1; i <= n; i++)
    {
        for(int j = i + 1; j <= n; j++)
        {
            if(ds[j] > ds[i])
            {
                ddif = ds[j] - ds[i];
            }
            else
            {
                ddif = ds[i] - ds[j];
            }
            dsuma = ddif + d[i] + d[j];
            if(dsuma > dmax)
            {
                dmax = dsuma;
            }
        }
    }
    fout<<dmax;

    return 0;
}