Syntax of the INFORMAT statement
Basically, the same as the format statement:

Informat statement INFORMAT variable-name(s) informat ... variable-name(s) informat ;

For more information see SL:R, Chapter 9, SAS Language Statements

Why does the INFORMAT statement belong before the INPUT statement?
Remember what I said was an important rule:
The characteristics of a variable are defined the first time the SAS compiler encounters the variable.