#include <algorithm>
#include <fstream>
#define PII pair<int, int>
#define x first
#define y second

using namespace std;

const int N=50005;

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

PII a[N];

int main()
{
    int n, m, i, maxs, sol=0;
    fin>>m>>n;
    for(i=1;i<=n;i++)  fin>>a[i].x>>a[i].y;
    sort(a+1, a+n+1);
    maxs=a[1].y;
    for(i=2;i<=n;i++)
    {
        if(a[i].x-a[i-1].x+a[i].y+maxs>sol) sol=a[i].x-a[i-1].x+a[i].y+maxs;
        if(a[i].y<maxs+a[i].x-a[i-1].x) maxs=maxs+a[i].x-a[i-1].x;
        else maxs=a[i].y;
    }
    fout<<sol;
    fin.close();
    fout.close();
}