include <iostream>

using namespace std;

int main() {
int a, b; cin>>b>>a;
int n; cin>>n;
int arr[100];
arr[1] = b; arr[2] = a;
for(int i = 3;i <= n; i++) arr[i] = 2*arr[i - 1] - arr[i - 2];
cout<<arr[n]<<endl;
return 0;
}