What is this test. What is my mistake
#include <cstdio>
#include <iostream>
#include <cmath>
using namespace std;
int main()
{
    int n;
    double r, a[128], b[128], P=0;
    scanf("%d%lf", &n, &r);
    for(int i=0; i<n; i++)
    {
            scanf("%lf%lf", &a[i], &b[i]);
            if(i>0) {
                    P+=sqrt((a[i]-a[i-1])*(a[i]-a[i-1])+(b[i]-b[i-1])*(b[i]-b[i-1]));
                    if(n==2) P+=sqrt((a[i]-a[i-1])*(a[i]-a[i-1])+(b[i]-b[i-1])*(b[i]-b[i-1]));
                    }
            if(i==n-1) P+=sqrt((a[i]-a[0])*(a[i]-a[0])+(b[i]-b[0])*(b[i]-b[0]));
    }
    P+=2*r*acos(-1.0);
    printf("%4.2lf\n", P);
}

how to do it for 0,001 seconds????