#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[8]={-1,0,1,0,1,1,-1,-1};
const ll dx[8]={0,-1,0,1,1,-1,1,-1};
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

ll read(string s){
  ll ret=0;
  ret+=stoll(s.substr(0,2));ret*=60;
  ret+=stoll(s.substr(3,2));
  return ret;
}
long double calc(ll n,ll k){
  long double ret=1;
  rep(i,k){
    ret*=(k-i);
    ret/=(n-1-i);
  }
  return ret/n;
}
int main(){
  ll n,l;cin >> n >>l;l*=60;
  vector<ll> v(n);
  rep(i,n){
    string s;cin >> s;
    v[i]=read(s);
  }
  if(accumulate(all(v),0LL)<=l){
    cout << n <<endl;return 0;
  }
  long double ans=0;
  rep(x,n){
    ll dp[n+1][l+1][n+1];
    rep(i,n+1)rep(j,l+1)rep(k,n+1)dp[i][j][k]=0;
    dp[0][0][0]=1;
    rep(i,n){
      rep(j,l+1){
        rep(k,n+1){
          if(!dp[i][j][k])continue;
          dp[i+1][j][k]+=dp[i][j][k];
          if(i==x)continue;
          if(v[i]+j<=l)dp[i+1][v[i]+j][k+1]+=dp[i][j][k];
        }
      }
    }
    rep(j,l){
      rep(k,n+1){
        if(dp[n][j][k]){
          ans+=calc(n,k)*dp[n][j][k];
        }
      }
    }
  }
  CST(10);
  cout << ans <<endl;
}