N,K=map(int,input().split())
T = list(map(int,input().split()))
fold = 0
m = T[0]
for i in range(1,K):
    if T[i] > m - 1:
        fold += (T[i] - (m-1)) * i
        m = T[i]
    else:
        fold += m - 1 - T[i]
        m = m - 1
min = N * 10**9
for i in range(1,N-K+1):
    if T[i-1] < m + K - 1 and T[i+K-1] < m:
        fold -= m + K - 1 - T[i-1]
        m = m - 1
        fold += m - T[i+K-1]
    if T[i-1] < m + K - 1 and T[i+K-1] == m:
        fold -= m + K - 1 - T[i-1]
        fold += K - 1
    if T[i-1] < m + K - 1 and T[i+K-1] > m:
        fold -= m + K - 1 - T[i-1]
        fold += (K - 1) * (T[i+K-1] - m + 1)
        m = T[i+K-1]
    else:
        fold = 0
        m = T[i]
        for j in range(i,i+K):
            if T[j] > m - 1:
                fold += (T[j] - (m-1)) * (j-i)
                m = T[j]
            else:
                fold += m - 1 - T[j]
                m = m - 1
    if fold < min:
        min = fold
print(min)