#include <cstdio>

using namespace std;

FILE* in = fopen("orase.in", "r");
FILE* out = fopen("orase.out", "w");

int main() {
  int m, n, maxsum = -9999999, maxdif = -9999999;
  fscanf(in, "%d %d", &m, &n);
  for(int i = 1;i <= n; i ++){
    int x,y;
    fscanf(in,"%d %d", &x, &y);
    if(x+y > maxsum)
      maxsum = x + y;
    if(y - x > maxdif)
      maxdif = y - x;
  }
  fprintf(out,"%d",maxsum+maxdif);
  return 0;
}