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)
DP=[[0]*L for c in range(N+1)]
DP[0][0]=1
for s in S:
    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]
dp=[[0]*(L+1) for c in range(N+1)]
for j in range(N):
    s=S[j]
    for c in range(N+1):
        for i in range(L):
            if c-1>=0 and i-s>=0:
                dp[c][i]=DP[c][i]-dp[c-1][i-s]
            else:
                dp[c][i]=DP[c][i]
    for c in range(N):
        ans+=sum(dp[c])*fact[c]*fact[N-c-1]
ans/=fact[N]
print(ans)