The Minus Equals assignment operator will assign a
new value to the variable in front of it: whatever
the variable's current value is - (subtract) the value 
after the operator.

For example:

int A = 10;                /* A is 10  */

A -= 4;                   /* A = A - 4;  So A is now 6. */

A -= 2;                   /* A = A - 2;  So A is now 4. */