from collections import deque
n = int(input())
B = list(map(int,input().split()))
ans = 0
left = deque()
need = deque()
for i,b in enumerate(B):

    if b == 0:
        if left:
            ans += i-left.popleft()
        else:
            need.append(i)
    else:
        for j in range(b-1):
            left.append(i)

        while left and need:
            ans += left.popleft()-need.popleft()
print(ans)