#include <cstdio>
#include <iostream>

using namespace std;
int main (void) {
    int n,z=0,b=1,m=n;
    cin>>n;
    while (n-b>0) {
     z=z+(n-b);
     n=n-b;
     b=b+1;
    }
    cout<<(z+n+1);
return 0;
}