import sys
readline=sys.stdin.readline

N,L=map(int,readline().split())
L*=60
S=readline().split()
for i in range(N):
    m,s=map(int,S[i].split(":"))
    S[i]=m*60+s
ans=0
fact=[1]
for i in range(1,N+1):
    fact.append(fact[-1]*i)
for j in range(N):
    dp=[[0]*L for c in range(N)]
    dp[0][0]=1
    for s in S[:j]+S[j+1:]:
        for c in range(N-1,0,-1):
            for i in range(L-1,s-1,-1):
                dp[c][i]+=dp[c-1][i-s]
    for c in range(N):
        ans+=sum(dp[c])*fact[c]*fact[N-c-1]
ans/=fact[N]
print(ans)