#include <fstream>
#include <algorithm>
using namespace std;

ifstream fi("orase.in");
ofstream fo("orase.out");
long long drum,maxl,L[50001],i,m,n;
pair <long long,long long> X[50001];
int main()
{
    fi>>m>>n;
    for(i=1;i<=n;i++)
        fi>>X[i].first>>X[i].second;
    sort(X+1,X+n+1);
    for(i=1;i<=n;i++)
        L[i]=X[i].second-X[i].first;
    maxl=L[1];
    drum=0;
    for(i=2;i<=n;i++)
    {
        drum=max(drum,X[i].first+X[i].second+maxl);
        if(maxl<L[i])
            maxl=L[i];
    }
    fo<<drum;
}