1 #include <iostream>
 2 using namespace std;
 3 int a(int c){
 4     if(c<=2){
 5         return c;
 6     }else{
 7         return a(c-1)+(c-2);
 8     }
 9 }
10 int main(int argc, char** argv) {
11     int c,k;
12     cin>>c;
13     cout<<a(c);
14     return 0;
15 }