C++ Randomness

#include <iostream>	

using namespace std;
		
		double pi = 3.14159 // Get a value of pi
		double diameter;
		double circumference = pi*diameter;

int main() {
	cout << "**************************\n";
	cout << "*Circumference Calculator*\n";
	cout << "**************************\n";
	cout << "\n";
	cout << "What's the diameter? ";
	cin >> diameter;
	cin.ignore();
	cout << "The circumference is approx. " << circumference << ".\n\n";
	cout << "Press Enter to exit.";
	cin.get();

}