z7

#include <iostream>
#include <math.h>
using namespace std;
float a;
int main(){
cout << "podaj liczbe: " << endl;
cin >> a;
while (a < 0){
    cout << "promien musi byc wiekszy od 0! wprowadz promien ponownie" << endl;
    cin >> a;
}
cout << "obwod kola to: " << 2*a*M_PI << endl;
cout << "pole kola to: " << a*a*M_PI << endl;
}