# include <fstream>
# include <algorithm>
# define DIM 50010
# define f first
# define s second
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
pair <int,int> s[DIM];
int m[DIM],i,n,m1,x,y,maxim,Maxim;
int main () {
    fin>>m1>>n;
    for(i=1;i<=n;i++){
        fin>>x>>y;
        s[i].s=y+x;
        s[i].f=x;
    }
    sort(s+1,s+n+1);
    for(i=n;i>=1;i--){
        if(s[i].s>maxim)
            maxim=s[i].s;
        m[i]=maxim;
    }
    for(i=1;i<n;i++){
        if(s[i].s+maxim-s[i].f>Maxim)
            Maxim=s[i].s+maxim-2*s[i].f;
    }
    fout<<Maxim<<"\n";
    return 0;
}