#include <cstdio>

using namespace std;

struct oras{
	long d,l;
}orase[50001];

long m,n;
long dif,maxim;

long abs (long x) {
	
	if(x>=0)
		return x;
	return -x;
}

int main()
{
	freopen("orase.in","r",stdin);
	freopen("orase.out","w",stdout);
	
	scanf("%ld%ld",&m,&n);
    
    for(int i=1;i<=n;i++)
    {
        scanf("%ld%ld",&orase[i].d,&orase[i].l);
    }
	
    for(int i=1;i<=n;i++)
    {
        for(int j=i+1;j<=n;j++)
        {
            dif=orase[i].l+abs(orase[j].d-orase[i].d)+orase[j].l;
            if(dif>maxim)
				maxim=dif;
        }
    }
    printf("%ld\n",maxim);
	return 0;
}