Why is my code incorrect? I'm trying to solve Theatre Square from codebreakers

[ChrisQ]

Theatre Square in the capital city of Berland has a rectangular shape with the size n × m meters. On the occasion of the city’s anniversary, a decision was taken to pave the Square with square granite flagstones. Each flagstone is of the size a × a.

What is the least number of flagstones needed to pave the Square? It’s allowed to cover the surface larger than the Theatre Square, but the Square has to be covered. It’s not allowed to break the flagstones. The sides of flagstones should be parallel to the sides of the Square.

Input
The input contains three positive integer numbers in the first line: n, m and a (1 ≤ n, m, a ≤ 109).

Output
Write the needed number of flagstones.

Sample 1
Input
6 6 4

Output
4

#include <bits/stdc++.h>
using namespace std;

int main(){
    double a, b, c, d;
    cin >> a >> b >> c;
    d = ceil(a/c)*ceil(b/c);
    cout << d << endl;


    return 0;
}

[ArielLeslie]

I’ve edited your code for readability. When you enter a code block into a forum post, please precede it with a separate line of three backticks and follow it with a separate line of three backticks to make it easier to read.

You can also use the “preformatted text” tool in the editor (</>) to add backticks around text.

See this post to find the backtick on your keyboard.
Note: Backticks (`) are not single quotes (').