def main():
    from math import sqrt,sin,cos,tan,ceil,radians,floor,gcd,exp,log,log10,log2,factorial,fsum
    from heapq import heapify, heappop, heappush
    from bisect import bisect_left, bisect_right
    from copy import deepcopy
    import copy
    import random
    from collections import deque,Counter,defaultdict
    from itertools import permutations,combinations
    from decimal import Decimal,ROUND_HALF_UP
    #tmp = Decimal(mid).quantize(Decimal('0'), rounding=ROUND_HALF_UP)
    from functools import lru_cache, reduce
    #@lru_cache(maxsize=None)
    from operator import add,sub,mul,xor,and_,or_,itemgetter
    INF = 10**18
    mod1 = 10**9+7
    mod2 = 998244353
    
    #DecimalならPython
    
    
    '''
    
    
    
    '''
    
    N = int(input())
    A = list(map(int, input().split()))
    
    G = [[] for _ in range(N)]
    memo = [set() for _ in range(N)]
    
    memo[0].add(-1)
    memo[N-1].add(-1)
    
    for i in range(1,N):
        if A[i-1]<=A[i]:
            memo[i-1].add(i)
            G[i].append(i-1)
        else:
            memo[i].add(i-1)
            G[i-1].append(i)
    
    Q = deque()
    
    for i in range(N):
        if len(memo[i]) == 0:
            Q.append(i)
    
    ans = 0
    while len(Q) > 0:
        pos = Q.popleft()
        
        A[pos] = min(A[pos+1],A[pos-1])-1
        memo[pos].add(pos+1)
        memo[pos].add(pos-1)
        G[pos+1].append(pos)
        G[pos-1].append(pos)
        
        ans += 1
        
        for nx in G[pos]:
            if pos in memo[nx]:
                memo[nx].remove(pos)
                if len(memo[nx]) == 0:
                    Q.append(nx)
    
    print(ans)
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
if __name__ == '__main__':
    main()