#include <cstdio>
#include <fstream>
#include <vector>

using namespace std;

#define N 1000005

int v[N];

int main ()
{

    ifstream in ("orase.in");
    int n,m;
    in>>m>>n;
    for(int i,j;n;--n){
        in>>i>>j;
        if(v[i]<j)
            v[i]=j;
    }
    int a=0;
    int mm=0;
    for(int i=0;i<=m;++i){
        if(mm<v[i]+i+a)
            mm=v[i]+i+a;
        if(a<v[i]-i)
            a=v[i]-i;
    }
    freopen ("orase.out","w",stdout);
    printf("%d\n",mm);

return 0;}