class BIT: #0-indexed def __init__(self, n): self.size = n self.tree = [0]*(n+1) self.depth = n.bit_length() self.n0 = 1< 0: s += self.tree[i] i -= i & -i return s def query(self,l,r): #a_l + ... + a_r 閉区間 return self.get_sum(r) - self.get_sum(l-1) def add(self, i, x): i += 1 while i <= self.size: self.tree[i] += x i += i & -i # self.element[i] += x #def get(self,i): return element[i] def bisect_left(self,w): #和が w 以上になる最小の index #w が存在しない場合 self.size を返す if w <= 0: return 0 x,k = 0,self.n0 for _ in range(self.depth): k >>= 1 if x+k <= self.size and self.tree[x+k] < w: w -= self.tree[x+k] x += k return x class interval_manager: #0-indexed def __init__(self, n, A=None): self.n = n self.isL = BIT(n) INIT_DATA() # グローバル関数でグローバル変数を初期化しちゃう if A is not None: assert len(A) == n self.val = A[::] self.nxt = list(range(1,n+1)) self.prv = list(range(-1,n)) for i in range(1,n+1): # isL[i] = 1 となるように初期化 self.isL.tree[i] = i&-i else: # 全て 0 で初期化 self.val = [0]*n self.nxt = [0]*n self.nxt[0] = n self.prv = [-1]*(n+1) self.prv[-1] = 0 self.isL.add(0,1) def left(self,i): # i を含む半開区間の左端 return self.isL.bisect_left(self.isL.get_sum(i)) def right(self,i): # i を含む半開区間の右端 return self.nxt[self.left(i)] def get(self,i): # A[i] の値を返す return self.val[self.left(i)] def _cut(self,L,i): # 区間 [L,nxt[L]) を i で切る. R = self.nxt[L] self.prv[R] = self.nxt[L] = i self.prv[i] = L self.nxt[i] = R self.val[i] = self.val[L] self.isL.add(i,1) def debug(self): print("sa ",sa) print("isL",[self.isL.query(i,i) for i in range(self.n)]) print("val",self.val) print("nxt",self.nxt) print("prv",self.prv) def update(self,L,R,x): # cut LL = self.left(L) if L != LL: self._cut(LL,L) RL = self.left(R-1) if R != self.nxt[RL]: self._cut(RL,R) ### connect if L and self.val[self.prv[L]] == x: L = self.prv[L] if R != self.n and self.val[R] == x: R = self.nxt[R] ### delete and add LL = L while True: nxt = self.nxt[L] DELETE(L,nxt,self.val[L]) L = nxt if L == R: break self.isL.add(L,-1) ADD(LL,R,x) self.nxt[LL] = R self.prv[R] = LL self.val[LL] = x ##################################### import sys readline = sys.stdin.readline D,Q = map(int,readline().split()) def INIT_DATA(): return def DELETE(L,R,x): return def ADD(L,R,x): global ans ans = max(ans,x*(sa[R]-sa[L])) a = [0]*Q b = [0]*Q for i in range(Q): a[i],b[i] = map(int,readline().split()) b[i] += 1 INF = 1<<60 sa = sorted(set([0]+a+b+[INF])) za = {v:i for i,v in enumerate(sa)} data = interval_manager(len(sa)) ans = 0 for l,r in zip(a,b): il = za[l] ir = za[r] data.update(il,ir,1) print(ans)