import sequtils,strutils,math
var
    N = stdin.readline.parseInt
    A = stdin.readline.split.map(parseInt)
    s = sum(A)
    ans = int.high
for i in 1..(sqrt(s.float64) + 0.00001).int:
    var cnt = 0
    for j in 1..i:
        if j - 1 <= A.high:
            cnt += max(0, A[j - 1] - j)
        if j != i and 2 * i - 1 - j < A.len:
            cnt += max(0, A[2 * i - 1 - j] - j)
    for j in (i * 2 - 1)..A.high:
        cnt += A[j]
    ans = min(ans,cnt)
echo ans