from itertools import zip_longest as zl
from math import factorial as f
N,L=map(int,input().split())
L*=60
M=[]
for s in input().split():
  a,b = map(int,s.split(':'))
  M+=[a*60+b]

def G(M,dp,l,u):
  m=len(M)//2
  if m==0:
    t=max(len(dp[0])-M[0],0)
    return [sum(d[t:]) for d in dp]
  A=G(M[m:],*D(M[:m],dp,l[:],u[:]))
  B=G(M[:m],*D(M[m:],dp,l[:],u[:]))
  return [a+b for a,b in zl(A,B,fillvalue=0)]

def D(M,d0,l,u):
  dp=[d[:] for d in d0]
  for m in M:
    k=len(dp)-1
    if l[k]+m<L: dp.append([0]*L)
    for n in range(k, -1, -1):
      if l[n]>=L-m: continue
      nl=l[n]+m
      l[n+1]=min(l[n+1],nl)
      nu=min(u[n]+m,L-1)
      u[n+1]=max(u[n+1],nu)
      dp[n+1][nl:nu+1]=[a+b for a,b in zip(dp[n+1][nl:nu+1], dp[n][l[n]:u[n]+1])]
  return dp, l, u

if sum(M)<=L:
  a=N
else:
  dp=[[0]*L]
  dp[0][0]=1
  l=[0]+[L]*N
  u=[0]*(N+1)
  a=sum(v*f(n)*f(N-n) for n,v in enumerate(G(M,dp,l,u),1))/f(N)
print(a)