Mathf.PI  

var radius : float = 5;

function Start() {
var perimeter : float = 2 * Mathf.PI * radius;
Debug.Log("The perimeter of the circle is: " + perimeter);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
public float radius = 5;
void Start() {
float perimeter = 2 * Mathf.PI * radius;
Debug.Log("The perimeter of the circle is: " + perimeter);
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

public radius as single = 5

def Start():
perimeter as single = ((2 * Mathf.PI) * radius)
Debug.Log(('The perimeter of the circle is: ' + perimeter))