n = int(input())
a = list(map(int,input().split())) + [0]*110
L = len(a)
ans = 1<<30
s = sum(a)
for i in range(1,L):
    if i*i > s: break
    r = 0
    for j in range(i):
        r += max(a[j]-j-1,0)
    for j in range(i,2*i-1):
        r += max(a[j]-(2*i-1-j),0)
    for j in range(2*i-1,L):
        r += a[j]
    ans = min(r,ans)
print(ans)