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;
}

Pascal :

var a,i,s:longint;
begin
read(a);
for i:=1 to a-1 do
s:=s+i;
write(s);
end.

Java :

import java.util.*;
public class Main{
	public static void main(String args[]){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int c = 0;			
		for(int i = 1;i<=n;i++){
			for(int j = 1;j<=i;j++){
				if(i!=j){
					c++;						
				}
			}
		}
		System.out.println(c);	
	}
}

Python :

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