#include<fstream> 
#include<cmath>
using namespace std; 
ifstream f("orase.in");
ofstream g("orase.out");
const int MAX_N=1000001;
	int i,m,n,d[MAX_N],l[MAX_N],smax,j;
	int main() 
	{f>>m>>n; 
smax=-int(2e9);
	for(i=1;i<=n;i++) 
	f>>d[i]>>l[i]; 
	for(i=1;i<n;i++) 
		for(j=i+1;j<=n;j++)
			if(abs(d[i]-d[j]) + l[i]+l[j]>smax) smax=abs(d[i]-d[j])+l[i]+l[j];
		g<<smax;
return 0; 
}