N = int(input())
B = list(map(int,input().split()))
ans = 0
L = []
for i in range(N):
    if B[i]==0:
        L.append(i)
    elif B[i]>1:
        d = B[i]-1
        while d>0 and L:
            j = L.pop()
            B[j] = 1
            ans += i-j
            d -= 1
        if d>0:
            B[i+1] += d
            ans += d
print(ans)