mod = 1000000007
eps = 10**-9


def main():
    import sys
    input = sys.stdin.readline

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

    ans = N - 1
    E = (N - B[-1])
    for i in range(M-2, -1, -1):
        ans = min(ans, B[0] - 1 + M / (M - i - 1) + E / (M - i - 1))
        E += (N - B[i])
    print(ans)


if __name__ == '__main__':
    main()