from collections import deque
n=int(input())
b=list(map(int,input().split()))
ans=0
q=deque()
for i in range(n):
    if b[i]==0:
        q.append(i)
    else:
        num=b[i]-1
        while num>0 and q:
            out=q.popleft()
            ans+=i-out
            num-=1
            b[i]-=1
            b[out]+=1
b=b[::-1]
p=deque()
for i in range(n):
    if b[i]==0:
        p.append(i)
    else:
        num=b[i]-1
        while num>0 and q:
            out=p.popleft()
            ans+=i-out
            num-=1
            b[i]-=1
            b[out]+=1
print(ans)