# たとえば00140
# 左から貪欲法で動かせばいい
# Bを見ていって、数字があったら、Aの左から埋めていく
# ABC004Dと近いかもしれない
# BiをAjからAkに配るときのムーブ数を出力するdefがあるといい

def move_to_A(i, j, k):
    if j <= i and k <= i:
        dist1 = abs(i-j)
        dist2 = abs(i-k)
        if dist1 == 0 and dist2 == 0:
            move = 0
        elif dist2 == 0:
            move = dist1*(dist1+1)//2
        else:
            move = dist1*(dist1+1)//2 - (dist2-1)*dist2//2
    elif i <= j and i <= k:
        dist1 = abs(k-i)
        dist2 = abs(j-i)
        if dist1 == 0 and dist2 == 0:
            move = 0
        elif dist2 == 0:
            move = dist1*(dist1+1)//2
        else:
            move = dist1*(dist1+1)//2 - (dist2-1)*dist2//2
    else:
        # つまり j < i and i < k
        dist1 = abs(i-j)
        dist2 = abs(k-i)
        move = dist1*(dist1+1)//2 + dist2*(dist2+1)//2
    return move

# test
# move_to_A(3, 2, 4)

N = int(input())
B = list(map(int, input().split()))

left = 0
right = 0
ans = 0
for i in range(N):
    if B[i] > 0:
        right = left + B[i] - 1
        calc = move_to_A(i, left, right)
        ans += calc
        left = right +1
        right = left
    #print(i, B[i], 'calc', calc, 'ans', ans, left, right)
print(ans)