/*
TASK:ots
LANG:C++
*/
# include <iostream.h>
int main ()
{
    int a[10000];
    int p,n;cin >> n;
    for ( int i=0;i<n;i++) cin >> a[i];
    p=0;
 for ( int i=n;i>=1;i--)
 {
     p=p+i-1;}
     cout << p;
    cout << endl;
    system ("PAUSE");
     return 0;
}

    
