#include <iostream>
using namespace std;
int main(int argc, char* argv[])
{
	int b1,b2,b3;
	int d;
	cin>>b1>>b2>>b3;
	double r=(double)(b3-b2)/(b2-b1);
	if (r!=(int)r){
		cout<<0<<endl;
		return 0;
	}
	d=b3-r*b2;
	int b4=r*b3+d;
	cout<<b4<<endl;
	return 0;
}