#include <stdio.h>
#include <algorithm>
using namespace std;

struct nod{int d,l;};
typedef nod dint;

dint A[1000000];

bool cmp(dint x,dint y)
{
if (x.d<=y.d) return 1;
return 0;
}

int main()
{
freopen("orase.in","r",stdin);
freopen("orase.out","w",stdout);

int n,m,i,j;
scanf("%d%d",&m,&n);

for (i=1;i<=n;i++) scanf("%d%d",&A[i].d,&A[i].l);
sort(A+1,A+n+1,cmp);
int S=0,Sm=-1;

for (i=1;i<n;i++) //printf("%d %d\n",A[i].d,A[i].l);
for (j=i+1;j<=n;j++)
{
if (A[j].l-A[j].d>A[i].l-A[i].d) i-j;
else
{
S = A[i].l+A[j].l+A[j].d-A[i].d;
if (S>Sm) Sm=S;
}
}
printf("%d",Sm);
}