"use strict";

var processData = function processData(lines) {
  var r = +lines[0];
  console.log(r * Math.PI * 2);
};

var i = '';
process.stdin.on('data', function (c) {
  return i += c;
});
process.stdin.on('end', function () {
  var _require = require('os'),
      EOL = _require.EOL;

  var lines = i.split(EOL);
  /*your input text, split by lines*/

  processData(lines);
});
