#include<fstream> 
using namespace std; 
ifstream f("orase.in");
ofstream g("orase.out");
int main() 
{int i,m,n,d[50000],l[50000],sol,smax,s1,s2,v[50000];
	f>>m>>n; 
sol=-int(2e9);
	for(i=1;i<=n;i++) 
	{f>>d[i]>>l[i]; v[d[i]]=l[i];}
sort(d+1,d+n+1);
	for(i=1;i<=n-1;i++) 
{s1=d[n]-d[i]; s2=v[d[n]]+v[d[i]]; smax=s1+s2; 
if(smax>sol) sol=smax; }	
g<<sol;
return 0; 
}