#include <iostream>
#include <cmath>
using namespace std;
float area(float a, float n) {
if (a < 0 ) //if the value is negative it is invalid
return -1;
float area = (a*a) / pow(2, n-1);
return area;
}
int main() {
float a = 20.0, n = 10.0;
cout << "Area : " << area(a, n);
}