#include <iostream>
using namespace std;
int a[10005];
int main(){
	int n,m,f,c;
	cin>>n;
	cin>>m;
	cin>>f;
	a[1]=n;
	a[2]=m;
	c=a[2]-a[1];
	for(int i=1;i<=f;i++){
		a[i]=a[i-1]+c;
	}
		cout<<a[f];
		return 0;
}