#include<cstdio>
using namespace std;
double P[30],A[30][30],comb=1.0;
int n,k;
int main(){
	int i,j;
	freopen("dezastru.in","r",stdin);
	scanf("%d%d",&n,&k);
	for(i=1;i<=n;i++)
		scanf("%lf",&P[i]);
	fclose(stdin);
	for(i=0;i<=n;i++)
		A[i][0]=1.0;
	for(i=1;i<=n;i++){
		for(j=1;j<=k;j++){
			A[i][j]=A[i-1][j]+A[i-1][j-1]*P[i];
		}
	}
	for(i=n-k+1;i<=n;i++)
		comb*=i;
	for(i=1;i<=k;i++)
		comb/=i;
	freopen("dezastru.out","w",stdout);
	printf("%lf\n",A[n][k]/comb);
	fclose(stdout);
	return 0;
}