Note 10. Stockholders’ Equity

Stock Option Activity

A summary of stock option activity for the three months ended April 30, 2018 is as follows:

						Weighted
				Weighted		average
				average		remaining		Aggregate
	Number			exercise		contractual		intrinsic
	of shares			price		term (in years)		value
Options outstanding at January 31, 2018		16,024,146		$	16.76		6.1	$	738,648,507
Options granted		175,000			77.88
Options exercised		(788,378	)		9.30
Options forfeited/cancelled		(153,551	)		4.00
Options outstanding at April 30, 2018		15,257,217		$	17.98		6.0	$	797,016,507
Options vested and exercisable at April 30, 2018		6,342,353		$	5.47		4.7	$	410,121,313
Options vested and exercisable at April 30, 2018 and expected to vest thereafter		15,257,217		$	17.98		6.0	$	797,016,507

During the three months ended April 30, 2018, we granted 175,000 stock options, respectively, under the 2013 Equity Incentive Plan (EIP). The weighted average grant-date fair value of options granted was $35.04 for the three months ended April 30, 2018.

As of April 30, 2018, there was $113.4 million in unrecognized compensation cost related to unvested stock options granted under the 2007 Stock Plan (2007 Plan), 2012 Equity Incentive Plan and 2013 EIP. This cost is expected to be recognized over a weighted average period of 3.9 years.

As of April 30, 2018, we had authorized and unissued shares of common stock sufficient to satisfy exercises of stock options.

The total intrinsic value of options exercised was approximately $48.4 million for the three months ended April 30, 2018.

Restricted Stock Units

A summary of restricted stock unit (RSU) activity for the three months ended April 30, 2018 is as follows:

	Unreleased restricted			Weighted average grant
	stock units			date fair value
Balance at January 31, 2018		2,901,736		$	38.14
RSUs granted		637,710			69.94
RSUs vested		(319,124	)		34.58
RSUs forfeited/cancelled		(62,247	)		39.70
Balance at April 30, 2018		3,158,075		$	44.89

During the three months ended April 30, 2018, we granted 637,710 RSUs under the 2013 EIP with a weighted-average grant date fair value of $69.94 .

As of April 30, 2018, there was a total of $134.0 million in unrecognized compensation cost related to unvested RSUs. This cost is expected to be recognized over a weighted-average period of approximately 2.6 years.

Stock-Based Compensation

Compensation expense related to share-based transactions, including equity awards to employees, consultants, and non-employee directors, is measured and recognized in the condensed consolidated financial statements based on fair value. The fair value of each option award is estimated on the grant date using the Monte Carlo simulation or Black-Scholes option-pricing model. The stock-based compensation expense is recognized using a straight-line basis over the requisite service periods of the awards, which is generally four to nine years. For RSUs, fair value is based on the closing price of our common stock on the grant date.

Our option-pricing model requires the input of subjective assumptions, including the fair value of the underlying common stock, the expected term of the option, the expected volatility of the price of our common stock, risk-free interest rates, and the expected dividend yield of our common stock. The assumptions used in our option-pricing model represent management’s best estimates. These estimates involve inherent uncertainties and the application of management’s judgment. If factors change and different assumptions are used, our stock-based compensation expense could be materially different in the future.

The following table presents the weighted-average assumptions used to estimate the grant date fair value of options granted during the periods presented:

	Three months ended April 30,
	2018		2017
Volatility	41%		44%
Expected term (in years)		6.35		6.35
Risk-free interest rate	2.73%		1.95% - 2.17%
Dividend yield	0%		0%

For the three months ended April 30, 2018 and 2017, we capitalized an immaterial amount of stock-based compensation as part of our internal-use software capitalization.