N = int(input())
stack = []
for i, x in enumerate(map(int, input().split())):
    if x > 0:
        stack.append([i, x])
stack = stack[::-1]

ans = 0
for i in range(N):
    ans += abs(i - stack[-1][0])
    stack[-1][1] -= 1
    if stack[-1][1] == 0:
        stack.pop()

print(ans)