【題解】AtCoder ABC 163A – Circle Pond

【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";
}
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