#include <fstream>

using namespace std;
int p[1000000],d[100000];
int main()
{
    ifstream in("orase.in");
    ofstream out("orase.out");
    int a=0,b=0,sol=0,t,n,c,i;
    in>>n>>c;
    for(i=1;i<=c;i++)
    {
        in>>p[i]>>d[i];
        t=a+p[i]+d[i];
        sol=max(sol,t);
        a=max(a,d[i]-p[i]);
    }
    out<<sol;
    return 0;
}