def f(x):
    a,b = map(int,x.split(":"))
    return a*60+b

n,L = map(int,input().split())
*s, = map(f,input().split())

fac = [1.0]
for i in range(1,n+1):
    fac.append(fac[-1]*i)

def mul(x):
    for i in range(n,0,-1):
        for j in range(L-1,x-1,-1):
            dp[i][j] += dp[i-1][j-x]
    
def div(x):
    for i in range(n):
        for j in range(L-x):
            dp[i+1][j+x] -= dp[i][j]

L *= 60
dp = [[0]*L for _ in range(n+1)]
dp[0] = [1]*L
for si in s:
    mul(si)

ans = 0
for si in s:
    div(si)
    for i in range(n):
        ans += dp[i][-1]*fac[i]*fac[n-i-1]/fac[n]
    mul(si)
print(ans)