n = int(input())
m = float(input())
q = [float(input()) for i in range(5)]
x = float(input())
if x*m % 2 == 1:
    Rmin = m*x
    for i in range(n-7):
        m = min(m, q[0])
        q.pop(0)
        q.append(x)
        x = float(input())
        if x*m % 2 == 1:
            Rmin = min(Rmin, x*m)
else:
    Rmin = m*x
    for i in range(n-7):
        m = min(m, q[0])
        q.pop(0)
        q.append(x)
        x = float(input())
        if x*m % 2 == 1:
            Rmin = min(Rmin, x*m)
        else:
            Rmin = -1
print(Rmin)