#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
long long d[1000001];
int main()
{
    long long m,n,i,x,y,first=999999999,maxx=0,s,last=-1;
    fin>>m>>n;
    for(i=1;i<=n;i++)
        {
            fin>>x>>y;
            d[x]=max(d[x],y);
            if(x<first)
                first=x;
            if(x>last)
                last=x;
        }
    s=d[first];
    for(i=first+1;i<=last;i++)
    {
        s++;
        if(d[i]!=0)
        {
            if(s+d[i]>maxx)maxx=s+d[i];
            if(s<d[i])s=d[i];
        }
    }
    if(first==last)
        fout<<0;
    else
    fout<<maxx;

    return 0;
}