题解 100% AC

#include<bits/stdc++.h>
using namespace std;
long long x,k,d,ans,a,b;
int main(){
	cin>>x>>k>>d;
	x=abs(x),a=floor(x*1.0/d);
	if(a>k)ans=x-k*d;
	else if(a==k)ans=x-k*d;
	else{
		b=x-a*d;
		if((k-a)%2==1)ans=min(b+d,abs(b-d));
		else ans=b;
	}
	cout<<ans;
    return 0;
}