#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <queue>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstring>
#include <cctype>
#include <cassert>
#include <limits>
#include <functional>
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
#if defined(_MSC_VER) || __cplusplus > 199711L
#define aut(r,v) auto r = (v)
#else
#define aut(r,v) __typeof(v) r = (v)
#endif
#define each(it,o) for(aut(it, (o).begin()); it != (o).end(); ++ it)
#define all(o) (o).begin(), (o).end()
#define pb(x) push_back(x)
#define mp(x,y) make_pair((x),(y))
#define mset(m,v) memset(m,v,sizeof(m))
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
using namespace std;
typedef vector<int> vi; typedef pair<int,int> pii; typedef vector<pair<int,int> > vpii; typedef long long ll;
template<typename T, typename U> inline void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if(x < y) x = y; }

int main() {
	int N; int L;
	while(~scanf("%d%d", &N, &L)) {
		L *= 60;
		vector<int> A(N);
		rep(i, N) {
			int M, S;
			scanf("%d:%d", &M, &S);
			A[i] = M * 60 + S;
		}
		if(accumulate(all(A), 0) <= L) {
			printf("%d\n", N);
			continue;
		}
		vector<double> dp0((N+1) * L), dp1 = dp0;
		dp0[0 * L + 0] = 1;
		rep(j, N) {
			int a = A[j];
			for(int k = j; k >= 0; -- k) rep(l, L) {
				double x = dp1[k * L + l];
				if(x < 1e-99) continue;
				double p = (k + 1) * 1. / (j + 1);
				dp1[k * L + l] = x * (1 - p);
				if(l + a < L)
					dp1[(k + 1) * L + (l + a)] += x * p;
			}
			for(int k = j; k >= 0; -- k) rep(l, L) {
				double x = dp0[k * L + l];
				if(x < 1e-99) continue;
				dp1[k * L + l] += x;
				double p = (k + 1) * 1. / (j + 2);
				dp0[k * L + l] = x * (1 - p);
				if(l + a < L)
					dp0[(k + 1) * L + (l + a)] += x * p;
			}
		}
		double ans = accumulate(all(dp1), 0.);
		printf("%.10f\n", ans);
	}
	return 0;
}