Coming !@#$%
Stupid PolarBear forget he's computer password, fortunately there is hint to help him retrieve it.
Hint Statement:
4x ≡ 1 mod (1e9 + 7).
Maybe x is the password we need.
PolarBear is very sad now.
He need your help to solve this equation.
hahahaha
Coming pengpeng
Review suggetstion：Please not use full name as commit content. Reviewee: @LaoshuBaby