I thought my algo is all right but got wa3&5.
Why?
Because I saw this in the statement: rounded to the integer
So,I wrote printf("%.0lf",a+0.49999999);
But it seemed to be wrong.
I checked again and again,finally I found the problem:
When you use %x.ylf,where x and y are numbers,the result is automatically rounded.
However,when using (int)a,it means FLOOR.
So,if you want to write rounded ans,there are 2 ways:
    printf("%d",(int)(a+0.499999999));
or  printf("%.0lf",a);

Nice! Now it works, thank you :0