#include<fstream>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
int j,n,t,i;
double s,x,dp[40][40];
int main(){
    fin>>n>>t;
    dp[0][0]=1;
    for(i=1;i<=n;++i){
        for(j=dp[i][0]=1,f>>x;j<=i;++j){
            dp[i][j]=dp[i-1][j]+x*dp[i-1][j-1];
        }
    }
    s=dp[n][t];
    for(i=t+1;i<=n;++i){
        s/=i;
    }
    for(i=1;i<=n-t;++i){
        s*=i;
    }
    fout<<s;
    return 0;
 }