【題目敘述】https://atcoder.jp/contests/abc163/tasks/abc163_a
#include <iostream>
#include <math.h>
using namespace std;
int n;
double ans;
int main() {
cin >> n;
ans = n * M_PI * 2;
cout << ans << "\n";
}
【題目敘述】https://atcoder.jp/contests/abc163/tasks/abc163_a
#include <iostream>
#include <math.h>
using namespace std;
int n;
double ans;
int main() {
cin >> n;
ans = n * M_PI * 2;
cout << ans << "\n";
}