N = int(input())
B = [int(b) for b in input().split()]

ans = 0
ball = 0
cnt = 0
for i in range(N):
    if B[i] == 0 and ball == 0:
        cnt += 1
    elif B[i] == 0:
        ans += ball
        ball -= 1
    elif cnt > 0:
        if B[i] == 1:
            cnt += 1
            continue
        ball += B[i]-1
        while ball > 0 and cnt > 0:
            if B[i-cnt] > 0:
                cnt -= 1
                continue
            ans += cnt
            cnt -= 1
            ball -= 1
    else:
        ans += ball
        ball += B[i]-1

print(ans)