#include <bits/stdc++.h>
using namespace std;
int main(){
    int a;
    cin>>a;
    if(a==2) cout<<1;
    else if(a==3) cout<<3;
    else if(a==4) cout<<6;
    else if(a==5) cout<<10;
    else if(a==6) cout<<15;
    else if(a==7) cout<<21;
    else if(a==8) cout<<28;
    else if(a==9) cout<<36;
    else if(a==10) cout<<45;
}
/**************************************************************
	Problem: 1332
	User: linmiaoling
	Language: C++
	Result: Accepted
	Time:11 ms
	Memory:2072 kb
****************************************************************/