#include <fstream>
#include <cmath>
#define N 1000000
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
long long n,m,s,smax,d[10001],l[N],i,j;
int main()
{
    f>>m>>n;
    for(i=1;i<=n;i++)
    f>>d[i]>>l[i];
    for(i=1;i<=n-1;i++)
       { for(j=i+1;j<=n;j++)
    s=fabs(d[i]-d[j])+l[i]+l[j];
    if(s>smax)
        smax=s;
        s=0;
    }
    g<<smax;

return 0;
}