$20!$ has 19 digits, the last 18 of which are 432902008176640000. What is the first digit?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
$20!$ has 19 digits, the last 18 of which are 432902008176640000. What is the first digit?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
2
$20!$ has 19 digits, the last 18 of which are 432902008176640000. What is the first digit?
train