N, M = map(int, input().split())
Bs = list(map(int, input().split()))

if M == 1:
    print(N - 1)
    exit()

s = 0
cnt = 0
x = N
for B in Bs[:0:-1]:
    cnt += 1
    s += Bs[-1] - B
    x = min(x, (M + s) / cnt)
    # print((M + s) / cnt)
    
# for _ in range(100):
#     mid = (lo + hi) / 2
#     mid_val = 0
#     for B in Bs:
#         mid_val += min(mid, Bs[-1] - B) + 1
#     if mid * N < mid_val:
#         lo = mid
#     else:
#         hi = mid
print(x + N - Bs[-1] + Bs[0] - 1)