#include <fstream>

using namespace std;
int l[50001], d[50001];
int main()
{
    ifstream f("orase.in");
    ofstream g("orase.out");
    int n,m,max = 0,i,a = 0 , b = 0;
    f>>m>>n;
    for(i = 0; i < n; i++){
       f>>d[i]>>l[i];
       if(d[i] > max)
        max = d[i];
    }
    for(i = 0; i < n; i++){
     if(d[i] + l[i] > a)
      a = d[i] + l[i];
     if(max - d[i] + l[i] > b)
      b = max - d[i] + l[i];
    }
    g<<a + b - max;
    return 0;
}