[Greedy] 게임을 만든 동준이

n = int(input())
arr = [int(input()) for x in range(n)]

answer = 0
arr = list(reversed(arr))
for i in range(len(arr)-1):
    if arr[i+1]>=arr[i]-1:
        answer += arr[i+1]-(arr[i]-1)
        arr[i+1] = arr[i]-1

print(answer)