Lab 10-31 (1) product of N positive numbers

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;


public class Untitled1 {
public static void main(String[] args ) throws IOException
{
BufferedReader keyboard= new BufferedReader(new InputStreamReader(System.in));
System.out.println("Enter number");
double a = 1;
double ans = 1;
while(a>=0)
{
a = Double.parseDouble(keyboard.readLine());
if (a<=0)
break;
ans = ans * a ;
}
System.out.println(ans);
}
}