I've spent two days now on this problem — it should be easy but I keep getting test 16 incorrect and I don't know why!

My code is pretty simple and straightforward:

int main(int argv, char* argc[]) {
    
        double m, n, a;
        while (scanf("%lf %lf %lf", &n, &m, &a) != EOF) {
        
            // Initiaslise tiles counters as 1 because we will
            // always have at least 1 tile.
            long long horiz = 1;
            long long vert  = 1;
        
            while (a < n) {
            
                n -= a;
                horiz ++;
            }
        
            while (a < m) {
            
                m -= a;
                vert ++;            
            }  
      
            printf("%d\n", (horiz * vert));
        }
  
return EXIT_SUCCESS;
}

Yet test 16 (m=1000000000, n=1000000000, n=192) returns a negative number! I get the following:

wrong answer 1st numbers differ — expected: '27126743055556', found: '-270385980'

Arrrgh why??