#include <bits/stdc++.h>

using namespace std;

// #define int long long
// #define x first
// #define y second
#define File(a) freopen(a".in", "r", stdin), freopen(a".out", "w", stdout)

inline void debug() { cerr << '\n'; }
template<typename Type, typename... Other>
inline void debug(const Type& x, const Other&... y) { cerr << x << ' '; debug(y...); }
#define DEBUG(a...) cerr << "[" << #a << "] = ", debug(a);

typedef long long LL;
typedef pair<int, int> PII;

const int N = 310;
const int INF = 0x3f3f3f3f;

template<typename Type>
inline void read(Type &res)
{
    res = 0;
    int ch = getchar(), flag = 0;
    while (!isdigit(ch)) flag |= ch == '-', ch = getchar();
    while (isdigit(ch)) res = (res << 3) + (res << 1) + (ch ^ 48), ch = getchar();
    res = flag ? -res : res;
}
template<typename Type, typename... Other>
inline void read(Type &res, Other&... y) { read(res), read(y...); }

int n, m;
double p[N], f[N][N], g[N];

signed main()
{
    read(n, m);
    for (int i = 0; i < n; i ++) scanf("%lf", &p[i]);
    f[m][0] = 1, g[m] = p[0];
    for (int i = m - 1; i >= 1; i --)
    {
        for (int j = 0; j < n; j ++)
        {
            if (j == 0) f[i][j] = 1;
            else f[i][j] = f[i][j - 1] * g[i + 1];
            g[i] += p[j] * f[i][j];
        } 
    }
    printf("%.4lf\n", g[1]);
    

    return 0;
}