import sys, re
from math import ceil, floor, sqrt, pi, factorial, gcd
from copy import deepcopy
from collections import Counter, deque, defaultdict
from heapq import heapify, heappop, heappush
from itertools import accumulate, product, combinations, combinations_with_replacement, permutations
from bisect import bisect, bisect_left, bisect_right
from functools import reduce
from decimal import Decimal, getcontext
def i_input(): return int(input())
def i_map(): return map(int, input().split())
def i_list(): return list(i_map())
def i_row(N): return [i_input() for _ in range(N)]
def i_row_list(N): return [i_list() for _ in range(N)]
def s_input(): return input()
def s_map(): return input().split()
def s_list(): return list(s_map())
def s_row(N): return [s_input for _ in range(N)]
def s_row_str(N): return [s_list() for _ in range(N)]
def s_row_list(N): return [list(s_input()) for _ in range(N)]
def lcm(a, b): return a * b // gcd(a, b)
def get_distance(x1, y1, x2, y2):
	  d = sqrt((x2 - x1) ** 2 + (y2 - y1) ** 2)
	  return d
def rotate(table):
   	  n_fild = []
   	  for x in zip(*table[::-1]):
   	  	  n_fild.append(x)
   	  return n_fild
sys.setrecursionlimit(10 ** 7)
INF = float('inf')
MOD = 10 ** 9 + 7
MOD2 = 998244353


###関数コピーしたか？###
def main():
   
   n = int(input())
   B = i_list()
   two = []
   for i in range(n):
   	  if B[i] >= 2:
   	  	  two.append(i)
   
   ans = 0
   for i in range(n):
   	  if B[i] == 0:
   	  	  point = bisect_right(two, i)
   	  	  num1 = INF
   	  	  num2 = INF
   	  	  if point == 0:
   	  	  	  num2 = abs(two[point-1] - i)
   	  	  elif 1 <= point < len(two):
   	  	  	  num1 = abs(two[point-1] - i)
   	  	  	  num2 = abs(two[point-1] - i)
   	  	  elif point == len(two):
   	  	  	  num1 = abs(two[point-1] - i)
   	  	  ans += min(num1, num2)
   	  	  if num1 == INF and num2 != INF:
   	  	  	  B[two[point-1]] -= 1
   	  	  	  if B[two[point-1]] == 0:
   	  	  	  	  two.pop(point-1)
   	  	  if num1 != INF and num2 == INF:
   	  	  	  B[two[point-1]] -= 1
   	  	  	  if B[two[point-1]] == 0:
   	  	  	  	  two.pop(point-1)
   	  	  if num1 != INF and num2 != INF:
   	  	  	  if num1 > num2:
   	  	  	  	  B[two[point-1]] -= 1
   	  	  	  	  if B[two[point-1]] == 0:
   	  	  	  	  	  two.pop(point-1)
   	  	  	  else:
   	  	  	  	  B[two[point-1]] -= 1
   	  	  	  	  if B[two[point-1]] == 0:
   	  	  	  	  	  two.pop(point-1)
   print(ans)
   	  	  
   
if __name__ == '__main__':
    main()