k = int(input())
n = int(input())
a = []
for i in range(n):
    a.append(int(input()))
ans = 0
last = n - 1
while last >= 0:
    full = a[last] // k
    ans += full * 2 * (last + 1)
    a[last] %= k
    if a[last] == 0:
        last -= 1
        continue
    cur = a[last]
    a[last] = 0
    ans += 2 * (last + 1)
    while cur < k and last >= 0:
        last -= 1
        if last == -1:
            break
        take = min(k - cur, a[last])
        cur += take
        a[last] -= take
        if cur == k:
            break
print(ans)