'use strict';
  
  var log = Math.log, sqrt = Math.sqrt;
  
  module.exports = function (x) {
  	if (isNaN(x)) return NaN;
  	x = Number(x);
  	if (x < 1) return NaN;
  	if (x === 1) return 0;
  	if (x === Infinity) return x;
  	return log(x + sqrt(x * x - 1));
  };