#include<bits/stdc++.h>
using namespace std;

#define REP(i,a,b) for(i=a;i<b;i++)
#define rep(i,n) REP(i,0,n)

#define mygc(c) (c)=getchar_unlocked()
#define mypc(c) putchar_unlocked(c)

#define ll long long
#define ull unsigned ll

#define double long double

void reader(int *x){int k,m=0;*x=0;for(;;){mygc(k);if(k=='-'){m=1;break;}if('0'<=k&&k<='9'){*x=k-'0';break;}}for(;;){mygc(k);if(k<'0'||k>'9')break;*x=(*x)*10+k-'0';}if(m)(*x)=-(*x);}
void reader(ll *x){int k,m=0;*x=0;for(;;){mygc(k);if(k=='-'){m=1;break;}if('0'<=k&&k<='9'){*x=k-'0';break;}}for(;;){mygc(k);if(k<'0'||k>'9')break;*x=(*x)*10+k-'0';}if(m)(*x)=-(*x);}
void reader(double *x){scanf("%Lf",x);}
int reader(char c[]){int i,s=0;for(;;){mygc(i);if(i!=' '&&i!='\n'&&i!='\r'&&i!='\t'&&i!=EOF) break;}c[s++]=i;for(;;){mygc(i);if(i==' '||i=='\n'||i=='\r'||i=='\t'||i==EOF) break;c[s++]=i;}c[s]='\0';return s;}
template <class T, class S> void reader(T *x, S *y){reader(x);reader(y);}
template <class T, class S, class U> void reader(T *x, S *y, U *z){reader(x);reader(y);reader(z);}
template <class T, class S, class U, class V> void reader(T *x, S *y, U *z, V *w){reader(x);reader(y);reader(z);reader(w);}

void writer(int x, char c){int s=0,m=0;char f[10];if(x<0)m=1,x=-x;while(x)f[s++]=x%10,x/=10;if(!s)f[s++]=0;if(m)mypc('-');while(s--)mypc(f[s]+'0');mypc(c);}
void writer(ll x, char c){int s=0,m=0;char f[20];if(x<0)m=1,x=-x;while(x)f[s++]=x%10,x/=10;if(!s)f[s++]=0;if(m)mypc('-');while(s--)mypc(f[s]+'0');mypc(c);}
void writer(double x, char c){printf("%.15Lf",x);mypc(c);}
void writer(const char c[]){int i;for(i=0;c[i]!='\0';i++)mypc(c[i]);}
void writer(const char x[], char c){int i;for(i=0;x[i]!='\0';i++)mypc(x[i]);mypc(c);}
template<class T> void writerLn(T x){writer(x,'\n');}
template<class T, class S> void writerLn(T x, S y){writer(x,' ');writer(y,'\n');}
template<class T, class S, class U> void writerLn(T x, S y, U z){writer(x,' ');writer(y,' ');writer(z,'\n');}
template<class T> void writerArr(T x[], int n){int i;if(!n){mypc('\n');return;}rep(i,n-1)writer(x[i],' ');writer(x[n-1],'\n');}

int N, L;
int S[120];

double brute(void){
  int i, j, k, ind[120], rem;
  double res = 0, all = 0;

  rep(i,N) ind[i] = i;
  do{
    rem = L;
    rep(i,N){
      res++;
      rem -= S[ind[i]];
      if(rem <= 0) break;
    }
    all++;
  }while(next_permutation(ind,ind+N));

  return res / all;
}


double dp[53][30000], ini[53][30000];
double solve(void){
  int i, j, k, m, p, mx = 61 * 60;
  double res = 0, pr, inv;

  rep(i,N+1) rep(j,L+mx) dp[i][j] = 0;
  dp[0][0] = 1;
  p = 0;
  rep(k,N){
    p++;
    for(i=p-1;i>=0;i--){
      pr = (double)(i+1) / (p+1);
      for(j=L+mx-1;j>=0;j--){
        dp[i+1][j+S[k]] += dp[i][j] * pr;
        dp[i][j] *= 1 - pr;
      }
    }
  }

  rep(m,N){
    rep(i,N+1) rep(j,L+mx) ini[i][j] = dp[i][j];
    rep(i,N){
      pr = (double)(i+1) / (N+1);
      inv = 1.0 / (1-pr);
      rep(j,L){
        ini[i][j] *= inv;
        ini[i+1][j+S[m]] -= ini[i][j] * pr;
      }
    }
    rep(i,N+1) rep(j,L) res += ini[i][j];
  }
  
  return res;
}

int main(){
  int i, j, k;
  double res;

  reader(&N,&L);
  assert(1 <= N && N <= 50);
  assert(1 <= L && L <= 300);
  L *= 60;
  rep(i,N){
    reader(&j,&k);
    S[i] = j * 60 + k;
    assert(0 <= j && j <= 59);
    assert(0 <= k && k <= 59);
  }

//  res = brute();
//  writerLn(res);
  res = solve();
  writerLn(res);

  return 0;
}