C++ :

#include<bits/stdc++.h>
using namespace std;
int main(){
    int a;
    cin>>a;
    if(a==2){
    	cout<<1<<endl;
	}
	else if(a==3){
		cout<<3<<endl;
	}
	else if(a==4){
		cout<<6<<endl;
	}
		else if(a==5){
		cout<<10<<endl;
	}
		else if(a==6){
		cout<<15<<endl;
	}
		else if(a==7){
		cout<<21<<endl;
	}
		else if(a==8){
		cout<<28<<endl;
	}
		else if(a==9){
		cout<<36<<endl;
	}
		else if(a==10){
		cout<<45<<endl;
	}
	return 0;
}

Python :

n = int(input())
t = 0
for i in range(1, n):
    t += i
print(t)