Atcoder ABC 139D

Atcoder ABC 139D

解法：

等差数列求和公式，记得开 $ long long $

CODE:

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>

using namespace std;

#define LL long long

LL n,ans;

int main() {
    scanf("%lld",&n);
    ans = n * (n - 1) / 2;
    printf("%lld \n",ans);
    //system("pause");
    return 0;
}