#!usr/bin/env python3

from collections import defaultdict, deque

from heapq import heappush, heappop

from itertools import permutations, accumulate

import sys

import math

import bisect

def LI(): return [int(x) for x in sys.stdin.readline().split()]

def I(): return int(sys.stdin.readline())

def LS():return [list(x) for x in sys.stdin.readline().split()]

def S():

    res = list(sys.stdin.readline())

    if res[-1] == "\n":

        return res[:-1]

    return res

def IR(n):

    return [I() for i in range(n)]

def LIR(n):

    return [LI() for i in range(n)]

def SR(n):

    return [S() for i in range(n)]

def LSR(n):

    return [LS() for i in range(n)]



sys.setrecursionlimit(1000000)

mod = 1000000007



def solve():

    n = I()

    print((n*(n-1)>>1))

    return



#Solve

if __name__ == "__main__":

    solve()

#!usr/bin/env python3

from collections import defaultdict, deque

from heapq import heappush, heappop

from itertools import permutations, accumulate

import sys

import math

import bisect

def LI(): return [int(x) for x in sys.stdin.buffer.readline().split()]

def I(): return int(sys.stdin.buffer.readline())

def LS():return [list(x) for x in sys.stdin.readline().split()]

def S():

    res = list(sys.stdin.readline())

    if res[-1] == "\n":

        return res[:-1]

    return res

def IR(n):

    return [I() for i in range(n)]

def LIR(n):

    return [LI() for i in range(n)]

def SR(n):

    return [S() for i in range(n)]

def LSR(n):

    return [LS() for i in range(n)]



sys.setrecursionlimit(1000000)

mod = 1000000007



def solve():

    n = I()

    print((n*(n-1)>>1))

    return



#Solve

if __name__ == "__main__":

    solve()

#!usr/bin/env python3
from collections import defaultdict, deque
from heapq import heappush, heappop
from itertools import permutations, accumulate
import sys
import math
import bisect


def LI():
    return [int(x) for x in sys.stdin.readline().split()]


def I():
    return int(sys.stdin.readline())


def LS():
    return [list(x) for x in sys.stdin.readline().split()]


def S():
    res = list(sys.stdin.readline())
    if res[-1] == "\n":
        return res[:-1]
    return res


def IR(n):
    return [I() for i in range(n)]


def LIR(n):
    return [LI() for i in range(n)]


def SR(n):
    return [S() for i in range(n)]


def LSR(n):
    return [LS() for i in range(n)]


sys.setrecursionlimit(1000000)
mod = 1000000007


def solve():
    n = I()
    print((n * (n - 1) >> 1))
    return


# Solve
if __name__ == "__main__":
    solve()

#!usr/bin/env python3
from collections import defaultdict, deque
from heapq import heappush, heappop
from itertools import permutations, accumulate
import sys
import math
import bisect


def LI():
    return [int(x) for x in sys.stdin.buffer.readline().split()]


def I():
    return int(sys.stdin.buffer.readline())


def LS():
    return [list(x) for x in sys.stdin.readline().split()]


def S():
    res = list(sys.stdin.readline())
    if res[-1] == "\n":
        return res[:-1]
    return res


def IR(n):
    return [I() for i in range(n)]


def LIR(n):
    return [LI() for i in range(n)]


def SR(n):
    return [S() for i in range(n)]


def LSR(n):
    return [LS() for i in range(n)]


sys.setrecursionlimit(1000000)
mod = 1000000007


def solve():
    n = I()
    print((n * (n - 1) >> 1))
    return


# Solve
if __name__ == "__main__":
    solve()

-    return [int(x) for x in sys.stdin.readline().split()] +    return [int(x) for x in sys.stdin.buffer.readline().split()] -    return int(sys.stdin.readline()) +    return int(sys.stdin.buffer.readline())