#include<stdio.h>
const char in[]="dezastru.in", out[]="dezastru.out";
const int N= 1<<5;
float v[N], a[N][N], sol;
int n, k, i, j;
 
int main()
    {
        freopen(in,"r",stdin);
        freopen(out,"w",stdout);
        scanf("%d%d", &n, &k);
        for(int i = 1 ; i <= n ; ++i)
            scanf("%f ", &v[i]);
 
        for( i = 0 ; i <= n ; ++i)
            a[ i ][ 0 ] = 1;
        for( i = 1 ; i <= n ; ++i)
            for( j = 1; j <= i ; ++j)
                a[ i ][ j ] = a[ i - 1 ][ j ] + v[ i ] * a[ i - 1][ j - 1];
        sol = a[ n ][ k ];
        for( i = k + 1; i <= n ; ++i) sol /= i;
        for( i = 1 ; i <= n - k ; ++i) sol *= i;
        printf("%f\n", sol);
        return 0;
}