/* template.cpp [[[ */
#include <bits/stdc++.h>
using namespace std;
#define get_macro(a, b, c, d, name, ...) name
#define rep(...) get_macro(__VA_ARGS__, rep4, rep3, rep2, rep1)(__VA_ARGS__)
#define rrep(...) get_macro(__VA_ARGS__, rrep4, rrep3, rrep2, rrep1)(__VA_ARGS__)
#define rep1(n) rep2(i_, n)
#define rep2(i, n) rep3(i, 0, n)
#define rep3(i, a, b) rep4(i, a, b, 1)
#define rep4(i, a, b, s) for (ll i = (a); i < (ll)(b); i += (ll)s)
#define rrep1(n) rrep2(i_, n)
#define rrep2(i, n) rrep3(i, 0, n)
#define rrep3(i, a, b) rrep4(i, a, b, 1)
#define rrep4(i, a, b, s) for (ll i = (ll)(b) - 1; i >= (ll)(a); i -= (ll)s)
#define each(x, c) for (auto &&x : c)
#define fs first
#define sc second
#define all(c) begin(c), end(c)
using ui = unsigned;
using ll = long long;
using ul = unsigned long long;
using ld = long double;
const int inf = 1e9 + 10;
const ll inf_ll = 1e18 + 10;
const ll mod = 1e9 + 7;
const ll mod9 = 1e9 + 9;
const int dx[]{-1, 0, 1, 0, -1, 1, 1, -1};
const int dy[]{0, -1, 0, 1, -1, -1, 1, 1};
template<class T, class U> void chmin(T &x, const U &y){ x = min<T>(x, y); }
template<class T, class U> void chmax(T &x, const U &y){ x = max<T>(x, y); }
struct prepare_ { prepare_(){ cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(12); } } prepare__;
ll in(){ ll x; cin >> x; return x; }
vector<ll> in(size_t n){ vector<ll> v(n); each(x, v) cin >> x; return v; }
/* ]]] */

ll T, n;
ll s[100010], t[100010];

int main(){
  cin >> T >> n;
  rep(i, n) cin >> s[i] >> t[i];
  s[n] = t[n] = inf_ll;
  
  ll ii = 0;
  ll res = 0;
  ll sum = 0;
  rep(i, n){
    while (t[ii] <= s[i] + T) sum += t[ii] - s[ii], ii++, chmax(res, sum);
    if (s[i] <= s[ii] && s[ii] < s[i] + T) chmax(res, sum + s[i] + T - s[ii]);
    sum -= t[i] - s[i];
  }
  
  reverse(s, s + n);
  reverse(t, t + n);
  rep(i, n) s[i] = -s[i];
  rep(i, n) t[i] = -t[i];
  ii = 0;
  sum = 0;
  rep(i, n){
    while (t[ii] <= s[i] + T) sum += t[ii] - s[ii], ii++, chmax(res, sum);
    if (s[i] <= s[ii] && s[ii] < s[i] + T) chmax(res, sum + s[i] + T - s[ii]);
    sum -= t[i] - s[i];
  }

  cout << res << endl;
}