Prada

#include <iostream>
#include <cmath>
using namespace std;

int main()
{
    int HA,HB,D;
    cin>>HA>>HB>>D;
    if(HA<HB)
        swap(HA,HB);
    cout<<abs((D*D+HB*HB-HA*HA)/(2*D));
    return 0;
}