The password must be 4-20 letters or numbers. What do you mean?

The number of symbols in the password you are required to set is between 4 and 20, and the symbols can only be numbers or letters. For example: 1234,1234567901234567890,123qwest, a 1b2c3, etc. These are all passwords that meet the requirements. But something similar to the following does not meet the requirements:123456789012345678901(too many digits), 12-34(- symbol is neither a letter nor a number)! @ # $% (these are not letters or numbers).