C++ :

#include<bits/stdc++.h>
using namespace std;
int main()
{
	int n,a,b;
	//freopen("book.in","r",stdin);
	//freopen("book.out","w",stdout);
	cin>>n>>a>>b;
	while(n-a>0){
		n-=a;
		a+=b;
	}
	cout<<n;
}