pi的求法 acos(-1.0)

pi=acos(-1.0)

https://www.luogu.org/problemnew/show/T4529

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>
#include <time.h>
#include <string>
#include <set>
#include <map>
#include <list>
#include <stack>
#include <queue>
#include <vector>
#include <bitset>
#include <ext/rope>
#include <algorithm>
#include <iostream>
using namespace std;
#define ll long long
#define minv 1e-6
#define inf 1e9
//#define pi 3.1415926536
#define E  2.7182818284
const ll mod=1e9+;//
const int maxn=1e5+;

int main()
{
    int n,m;
    double pi=acos(-1.0);
    scanf("%d%d",&n,&m);
    printf("%.*f",m,pi*n);
    return ;
}