排列组合公式

1.在1-n中选取3个不同的数组成不全等的三角形

n==3   ans=0;

n>=4时  n为奇    (n-1)*(n-3)*(2n-1)/24

              n为偶    n*(n-2)*(2n-5)/24

#include<bits/stdc++.h>
#define ll long long
#define rep(i,x,y) for(register int i=x;i<=y;i++)
using namespace std;
ll n,ans;
int main(){
    freopen("count.in","r",stdin);
    freopen("count.out","w",stdout);
    scanf("%lld",&n);
    if(n==3) printf("0");
    if(n==4) printf("1");
    else 
       if(n%2) printf("%lld\n",(n-1)*(n-3)*(2*n-1)/24);
       else printf("%lld\n",(n-2)*(2*n-5)*n/24);    
    return 0;
}