#include<stdio.h>
struct Rectangle
{
    float length;
    float  width;
};
float calculateArea(struct Rectangle rect)
{
    if(rect.length<0||rect.width<0)
    {
        return -1;
    }
    return rect.length*rect,width;
}
int main()
{
    struct Rectangle rect;
    scanf("%f",&rect.length);
    scanf("%f",&rect.width);
    float area=calculateArea(rect);
    if(area<0)
    {
        printf("%.2f\n",area);
    }
    return 0;
}