0 / main

#include <iostream>
#include <math.h>
#include <conio.h>
/* run this program using the console pauser or add your own getch, system("pause") or input loop */
double r,L;
int main(int argc, char** argv) {
	std::cout<<"\nr=";
	std::cin>>r;
	L=2*M_PI*r;
	std::cout<<"L="<<L;
	getch();
	
	return 0;
}