Dividing an integer by an integer yields an integer. Try this:
#define BILLION 1E9
And don’t use a semicolon at the end of the line. #define is a preprocessor directive, not a statement, and including the semicolon resulted in BILLION being defined as 1000000000L;, which would break if you tried to use it in most contexts. You got lucky because you used it at the very end of an expression and outside any parentheses.