#include <fstream>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");

int mod(int a, int b)
{
    if(a > b) return a-b;
    else if(a < b ) return b-a;
    else if(a == b) return 0;
    else fout << "\n\nERROR\n\n";
}

int main()
{
    int N, C, i, j;
    int P[100000], D[100000];
    fin >> N >> C;
    for(i = 1; i <= C; i++)
    {
        fin >> P[i] >> D[i];
    }

    int Sol = 0, L = 0;
    for(i = 1; i < N; i++)
    {
        for(j = i+1; j <= N; j++)
        {
            L = D[i] + D[j] + mod(P[j], P[i]);
            if(L > Sol) Sol = L;
        }
    }

    fout << Sol;

    return 0;
}