#include<iostream>#include<stdio.h>using namespace std;typedef long long int64;int64 dp[ 51 ][ 51 ];int a[ 55 ],b[ 55 ];int main(){ int n,k; int64 tp; double sum; dp[ 0 ][ 0 ] = 1; dp[ 1 ][ 0 ] = dp[ 1 ][ 1 ] = 1; for(int i=2;i<=50;i++) for(int j=0;j<=i;j++) { if( j )dp[i][j] = dp[i-1][j-1] + dp[ i-1 ][j]; else dp[i][0] = 1; } while( scanf("%d%d",&n,&k) && n != -1 ) { tp = 0; for(int i=0;i<n;i++) { scanf("%d",&a[i]); tp += a[i]; } for(int i=0;i<n;i++) { scanf("%d",&b[i]); } sum = dp[ tp ][ k ]; tp = 1; for(int i=0;i<n;i++) { tp *= dp[ a[i] ][ b[i] ]; } printf("%.5lfn",tp/sum); }}
