#include<fstream>
#include<algorithm>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
long n,m,sum,max1,i;
struct st
{ 
	int d,l;
};
st dp[50002];
int comp(st a,st b)
{  
	return (a.d<b.d);
}
int main()
{  
	f>>m>>n;
    for(i=1;i<=n;i++)
        f>>dp[i].d>>dp[i].l;
    sort(dp+1,dp+n+1,comp);
    max1=dp[1].l-dp[1].d;
    for(i=2;i<=n;i++)
    {   
		if(sum<dp[i].l+dp[i].d+max1)
            sum=dp[i].l+dp[i].d+max1;
        if(max1<dp[i].l-dp[i].d)
            max1=dp[i].l-dp[i].d;
    }
    g<<sum;
    return 0;
}