inp = input()
global arr
arr = [int(x) for x in inp.split()]
inp = input()
global arr1
arr1 = [int(x) for x in inp.split()]
def search(i, real, req,fill):
    if i == arr[0] - 2:
        help = arr1[-1]
        if fill > 0:
            req += real*(arr[1] - fill)
            real += (i + 1)*2 + 2
            help -= arr[1] - fill
        while help > arr[1]:
            req += real*(arr[1])
            real += (i + 1)*2 + 2
            help -= arr[1]
        req += real*(help)
        return req
    else:
        req += max(real,1)*(i+1)
        if fill + arr1[i] <= arr[1]:
            return min(search(i+1,real+1,req,fill+arr[i]),search(i+1,real+2*i+2,req,0))
        else:
            return search(i,real+2*i+2,req,0)
print(search(0,0,0,0))