#include <iostream>
#include <fstream>
#include <cstring>
#include <algorithm>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
struct oras{int Di,Li;}o[50001];
/*int main()
{
    int n;
    cin>>n;

    long long nr=n*n-(n-1)-(n/2*(n/2-1));

    return 0;
}*/

bool sortc(oras o1,oras o2)
{
    return (o1.Di<o2.Di);
}
int main()
{
    long long M,N,dmax=-20000000000;
    fin>>M>>N;

    for(int i=1;i<=N;i++)
        fin>>o[i].Di>>o[i].Li;
    sort(o+1,o+N+1,sortc);

    for(int i=2;i<=N;i++)
    {
        long long d=o[i].Di-o[i-1].Di+o[i].Li+o[i-1].Li;
        if(d>dmax)
            dmax=d;
    }
    fout<<dmax;
}