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#include <bits/stdc++.h>
#define ll long long
#define llu long long unsigned
#define ld long double
#define fr(i,n) for(int i=0;i<n;i++)
#define watch(x) cout<<(#x)<<"=="<<(x)<<endl
#define ft first
#define sc second
#define mp make_pair
#define pb push_back
#define vi vector<int>
#define pii pair<int,int>
#define P 1000000007llu
#define N 5005
#define LC 262144
using namespace std;

int n,T;
double p[N], E[N], f[N][N];

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    cin>>n>>T;
    fr(i,n) {
        cin>>p[i];
        E[i]=2.0*p[i]-1.0, f[n][i]=0.0, f[i][0]=1.0;
       // watch(p[i]); watch(E[i]); watch(n); watch(i); watch(f[n][i]); watch(f[i][0]);
    }
    f[n][0]=1.0;
    sort(E,E+n); sort(p,p+n);
    int it=(int)(lower_bound(E,E+n,0.5)-E);
    if(n-it<T) it=n-T;
   /* fr(i,n) watch(E[i]);
    watch(it);*/
    for(int i=n-1;i>=it;i--) for(int t=1;t<=T;t++) {
        f[i][t]=p[i]*f[i+1][t-1]+(1.0-p[i])*f[i+1][t];
        //watch(i); watch(t); watch(f[i][t]); watch(f[i+1][t-1]); watch(f[i+1][t]);
    }
    if(f[it][T]<=1e-6) cout<<"0\n";
    else cout<<f[it][T]<<'\n';
    return 0;
}