#include <cstdio>
#include <algorithm>

const int MAXN = 5e4;

struct oras {
  int d, l;
  bool operator <(const oras &aux) const {
    return d < aux.d;
  }
} v[MAXN];

inline int max(int a, int b) {
  return a > b ? a : b;
}

int main() {
  int n, m, dmax, dfmax, dfind;
  FILE *f = fopen("orase.in", "r");
  fscanf(f, "%d%d", &m, &n);
  for (int i = 0; i < n; ++i) {
    fscanf(f, "%d%d", &v[i].d, &v[i].l);
  }
  fclose(f);
  std::sort(v, v + n);
  dmax = dfind = 0;
  dfmax = v[0].l - v[0].d;
  for (int i = 1; i < n; ++i) {
    dmax = max(dmax, v[i].l + v[dfind].l + v[i].d - v[dfind].d);
    if (v[i].l - v[i].d > dfmax) {
      dfmax = v[i].l - v[i].d;
      dfind = i;
    }
  }
  f = fopen("orase.out", "w");
  fprintf(f, "%d\n", dmax);
  fclose(f);
  return 0;
}