A company recently discovered that a former employee had been embezzling money. The employee was responsible for depositing checks received by the company into its bank account. It was discovered that the employee had been altering the amount of the checks before depositing them, and then taking the difference for personal use. The company estimated that the employee had been stealing an average of $500 per week for the past year. If the company lost a total of $26,000 due to the employee's theft, how many checks had the employee altered?