#include<bits/stdc++.h>
using namespace std;
using ld=long double;
const int N=310;
ld p[N];
ld qpow(ld x,int k){
	ld res=1;
	while(k){
		if(k&1)res*=x;
		x*=x,k>>=1;
	}
	return res;
}
int main(){
	int n,m;
	cin>>n>>m;
	for(int i=0;i<n;++i)
		cin>>p[i];
	ld ans=0,ans0;
	for(int i=1;i<=m;++i){
		ans0=ans,ans=0;
		for(int j=0;j<n;++j)
			ans+=p[j]*qpow(ans0,j);
	}
	cout<<fixed<<setprecision(4)<<ans;
	return 0;
}