import math
import sys
readline=sys.stdin.readline

N,L=map(int,readline().split())
L*=60
S=[]
for s in readline().split():
    mm,ss=map(int,s.split(":"))
    S.append(mm*60+ss)
dp=[[0]*18001 for i in range(N+1)]
dp[0][0]=1
ans=0
S_sum=sum(S)
if L>=S_sum:
    ans=N
else:
    for s in S:
        for i in range(N,0,-1):
            for j in range(18000,s-1,-1):
                dp[i][j]+=dp[i-1][j-s]
    for s in S:
        dp_=[[dp[i][j] for j in range(18001)] for i in range(N+1)]
        for i in range(1,N+1):
            for j in range(s,18001):
                dp_[i][j]-=dp_[i-1][j-s]
        for i in range(N):
            for j in range(L):
                ans+=math.factorial(i)*dp_[i][j]*math.factorial(N-i-1)
    ans/=math.factorial(N)
print(ans)