自己推的式子，应该可能大概是对的吧？

#include<bits/stdc++.h>
using namespace std;
int main(){
	long long n;
	cin>>n;
	long long ans=(n-1)*(n-2)*(n-3)/6;
	cout<<ans;
	return 0;
}
/*
--------------------
input1:
3

output1:
0
--------------------
input2:
114

output2:
234136
--------------------
input3:
1145

output3:
248878344
--------------------
input4:
11451

output4:
250121723400
--------------------
input5:
1000000

output5:
166665666668499999
--------------------
*/

欢迎hack