#include<iostream> // cin\cout\endl #include<cstdio> //scanf()\printf() #include<cstring> // strcpy()\strcat()\strcmp()\strlen()\memset() #include<cmath> //sqrt()\pow()\abs()\ceil()\floor()\max()\min() using namespace std; int main(){ int s=1,b,n=1; cin>>b; for(int i=3;i<=b;i++){ n++; s+=n; } cout<<s; return 0; }