#include <cstdio>
#include <algorithm>

using namespace std;

int n , m;

int main()
{
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
    scanf("%d%d",&n,&m);
    int ma1 = 0,ma2 = 0,a,b;
    for(int i = 1 ; i <= m ; i++)
    {
        scanf("%d%d",&a,&b);
        ma1 = max(ma1,a+b);
        ma2 = max(ma2,b-a);
    }
    printf("%d",ma1+ma2);
    return 0;
}