#include <stdio.h>   
  
long i, j, k, l, a, n;   
double s, x[32][32], c[32][32], p[32];   
  
int main(){   
freopen("dezastru.in", "rt", stdin);   
freopen("dezastru.out", "wt", stdout);   
scanf("%ld %ld",&n, &k);   
for (i=1; i<=n; i++) scanf("%lf", &p[i]);   
x[1][1]=p[1];   
c[1][1]=c[1][0]= x[1][0] = 1;   
for (i=2; i<=n; i++){   
        x[i][0]=c[i][0]=1;   
        for (j=1; j<=k && j<=i; j++) {  
            x[i][j]=x[i-1][j]+x[i-1][j-1]*p[i];   
            c[i][j]=c[i-1][j-1]+c[i-1][j];   
            }   
        }   
printf("%.6lf\n", x[n][k]/c[n][k]);   
return 0;   
}