You can not use (read) the variable that was not initialized first. You should assign the value to the variable before reading it.
Example (incorrect usage):
x = 1; 
z = x + y; // wrong, y is not initialized
Correct usage would look like this:
 
  x = 1; //
  initialize x 
  y = 2; //
  initialize y 
z = x + y; // correct, both x and y are initialized