"use strict";
var __importStar = (this && this.__importStar) || function (mod) {
    if (mod && mod.__esModule) return mod;
    var result = {};
    if (mod != null) for (var k in mod) if (Object.hasOwnProperty.call(mod, k)) result[k] = mod[k];
    result["default"] = mod;
    return result;
};
Object.defineProperty(exports, "__esModule", { value: true });
var fs = __importStar(require("fs"));
var input = fs.readFileSync('/dev/stdin', 'utf8').split('\n');
var n = input[0];
console.log("" + 2 * 3.14 * parseInt(n, 10));
