from sys import stdin
from bisect import bisect

def main():
    N,K = [int(i) for i in input().split()]
    C = [0] + [int(i) for i in stdin.readline().split()]
    s = [0]*(N+1)
    for i in range(N):
        C[i+1] += i
        s[i+1] = s[i] + C[i+1]
    db = []
    for i in range(1,K+1):
        db.insert(bisect(db,C[i]),C[i])
    maxi = db[-1]
    mine = maxi * K - (s[K] - s[0])
    for i in range(1,N-K+1):
        if C[i] != C[i+K]:
            db.remove(C[i])
            db.insert(bisect(db,C[i+K]),C[i+K])
        maxi = db[-1]
        m = s[i+K] - s[i]
        e = maxi * K - m
        if e < mine:
            mine = e
    print(mine)    
main()