#include <bits/stdc++.h>

int n, m ;

int main() {
  freopen("orase.in", "r",stdin) ;
  freopen("orase.out", "w",stdout) ;
  int i, l, d, max1(1 << 31), max2(1 << 31) ;
  scanf("%d %d", &m, &n) ;
  for (i = 1 ; i <= n ; ++ i) {
    scanf("%d %d", &l, &d) ;
    max1 = std::max(max1, l + d) ;
    max2 = std::max(max2, d - l) ;
  }
  printf("%d", max1 + max2) ;
  return 0 ; }