import sys input = sys.stdin.buffer.readline sys.setrecursionlimit(10 ** 7) inf = 10**20 class SegTree(object): def __init__(self, N, op_data, u_data): self._n = N self.log = (N-1).bit_length() self.size = 1 << self.log self.op = op_data self.e = u_data self.data = [u_data] * (2 * self.size) # self.len = [1] * (2 * self.size) def _update(self, i): self.data[i] = self.op(self.data[i << 1], self.data[i << 1 | 1]) def initialize(self, arr=None): """ segtreeをarrで初期化する。len(arr) == Nにすること """ if arr: for i, a in enumerate(arr, self.size): self.data[i] = a for i in reversed(range(1, self.size)): self._update(i) # self.len[i] = self.len[i << 1] + self.len[i << 1 | 1] def update(self, p, x): """ data[p] = x とする (0-indexed)""" p += self.size self.data[p] = x for i in range(1, self.log + 1): self._update(p >> i) def get(self, p): """ data[p]を返す """ return self.data[p + self.size] def prod(self, l, r): """ op_data(data[l], data[l+1], ..., data[r-1])を返す (0-indexed) """ sml = self.e smr = self.e l += self.size r += self.size while l < r: if l & 1: sml = self.op(sml, self.data[l]) l += 1 if r & 1: r -= 1 smr = self.op(self.data[r], smr) l >>= 1 r >>= 1 return self.op(sml, smr) def all_prod(self): """ op(data[0], data[1], ... data[N-1])を返す """ return self.data[1] def max_right(self, l, func): """ func(l, l+1, ..., r-1) = True, func(l, l+1, ..., r-1, r) = Falseとなる r を返す """ if l == self._n: return self._n l += self.size sm = self.e while True: while l % 2 == 0: l >>= 1 if not func(self.op(sm, self.data[l])): while l < self.size: l <<= 1 if func(self.op(sm, self.data[l])): sm = self.op(sm, self.data[l]) l += 1 return l - self.size sm = self.op(sm, self.data[l]) l += 1 if (l & -l) == l: break return self._n def min_left(self, r, func): """ func( l, l+1, ..., r-1) = True, func(l-1, l, l+1, ..., r-1) = Falseとなる l を返す """ if r == 0: return 0 r += self.size sm = self.e while True: r -= 1 while r > 1 and r & 1: r >>= 1 if not func(self.op(self.data[r], sm)): while r < self.size: r = r << 1 | 1 if func(self.op(self.data[r], sm)): sm = self.op(self.data[r], sm) r -= 1 return r + 1 - self.size sm = self.op(self.data[r], sm) if (r & -r) == r: break return 0 class LazySegTree(SegTree): def __init__(self, N, op_data, u_data, op_lazy, u_lazy, op_merge): super().__init__(N, op_data, u_data) self.composition = op_lazy self.mapping = op_merge self.id = u_lazy self.lazy = [u_lazy] * self.size def _all_apply(self, i, F): # self.data[i] = self.mapping(F, self.data[i], self.len[i]) self.data[i] = self.mapping(F, self.data[i]) if i < self.size: self.lazy[i] = self.composition(F, self.lazy[i]) def _push(self, i): self._all_apply(i << 1, self.lazy[i]) self._all_apply(i << 1 | 1, self.lazy[i]) self.lazy[i] = self.id def update(self, p, x): """ data[p] = x とする (0-indexed)""" p += self.size for i in reversed(range(1, self.log + 1)): self._push(p >> i) self.data[p] = x for i in range(1, self.log + 1): self._update(p >> i) def apply(self, p, F): """ data[p]にFを作用させる(data[p] = op_merge(F, data[p])とする, 0-indexed) """ p += self.size for i in reversed(range(1, self.log + 1)): self._push(p >> i) # self.data[p] = self.mapping(F, self.data[p], self.len[p]) self.data[p] = self.mapping(F, self.data[p]) for i in range(1, self.log + 1): self._update(p >> i) def range_apply(self, l, r, F): """ i = l, l+1, ..., r-1 について、Fを作用させる(op_merge(F, data[i]), 0-indexed) """ if l == r: return l += self.size r += self.size for i in reversed(range(1, self.log + 1)): # too->down if ((l >> i) << i) != l: self._push(l >> i) if ((r >> i) << i) != r: self._push((r - 1) >> i) l2, r2 = l, r while l < r: if l & 1: self._all_apply(l, F) l += 1 if r & 1: r -= 1 self._all_apply(r, F) l >>= 1 r >>= 1 l, r = l2, r2 for i in range(1, self.log + 1): if ((l >> i) << i) != l: self._update(l >> i) if ((r >> i) << i) != r: self._update((r - 1) >> i) def get(self, p): """ data[p]を返す """ p += self.size for i in reversed(range(1, self.log + 1)): self._push(p >> i) return self.data[p] def prod(self, l, r): """ op_data(data[l], data[l+1], ..., data[r-1])を返す (0-indexed) l == rの時は単位元u_dataを返す """ if l == r: return self.e l += self.size r += self.size for i in reversed(range(1, self.log + 1)): if ((l >> i) << i) != l: self._push(l >> i) if ((r >> i) << i) != r: self._push(r >> i) sml = self.e smr = self.e while l < r: if l & 1: sml = self.op(sml, self.data[l]) l += 1 if r & 1: r -= 1 smr = self.op(self.data[r], smr) l >>= 1 r >>= 1 return self.op(sml, smr) def max_right(self, l, func): """ func(l, l+1, ..., r-1) = True, func(l, l+1, ..., r-1, r) = Falseとなる r を返す """ if l == self._n: return self._n l += self.size for i in reversed(range(1, self.log + 1)): self._push(l >> i) sm = self.e while True: while l % 2 == 0: l >>= 1 if not func(self.op(sm, self.data[[l]])): while l < self.size: self._push(l) l <<= 1 if func(self.op(sm, self.data[l])): sm = self.op(sm, self.data[l]) l += 1 return l - self.size sm = self.op(sm, self.data[l]) l += 1 if (l & -l) == l: break return self._n def min_left(self, r, func): """ func( l, l+1, ..., r-1) = True, func(l-1, l, l+1, ..., r-1) = Falseとなる l を返す """ if r == 0: return 0 r += self.size for i in reversed(range(1, self.log + 1)): self._push((r - 1) >> i) sm = self.e while True: r -= 1 while r > 1 and r & 1: r >>= 1 if not func(self.op(self.data[r], sm)): while r < self.size: self._push(r) r = r << 1 | 1 if func(self.op(self.data[r], sm)): sm = self.op(self.data[r], sm) r -= 1 return r + 1 - self.size sm = self.op(self.data[r], sm) if (r & -r) == r: break return 0 def show(self): res = [self.get(i) for i in range(self._n)] print(*res) """ 遅延セグ木(ac-library移植) op_data(d_L, d_R) : d_Lとd_Rの二項演算, dataを返す op_lazy(lz_new, lz_orig) : lz_origにlz_newを作用させる, lazyを返す op_merge(lz, d) : dにlzを作用させる, dataを返す """ u_lazy = None def op_lazy(nw, od): if nw is None: return od return nw def op_merge(lz, d): if lz is None: return d return lz D, Q = map(int, input().split()) AB = tuple(tuple(map(int, input().split())) for _ in range(Q)) day = set() day.add(-1) day.add(D+5) for a, b in AB: day.add(a) day.add(a-1) day.add(b) day.add(b+1) size = len(day) dtoi = {d: i for i, d in enumerate(sorted(day))} minseg = LazySegTree(size, min, inf, op_lazy, u_lazy, op_merge) maxseg = LazySegTree(size, max, -inf, op_lazy, u_lazy, op_merge) minseg.initialize() maxseg.initialize() ans = 0 for a, b in AB: l = dtoi[a] r = dtoi[b] + 1 vmin = min(a, minseg.prod(l-1, r+1)) minseg.range_apply(l, r, vmin) vmax = max(b, maxseg.prod(l-1, r+1)) maxseg.range_apply(l, r, vmax) ans = max(ans, vmax - vmin + 1) print(ans) # print(ans, (l, r), (vmin, vmax))