#include<bits/stdc++.h>
#define rep(x,y,z) for(int x=y;x<=z;x++)
#define drep(x,y,z) for(int x=y;x>=z;x--)
#define ull unsigned long long
#define ll long long
using namespace std;
inline int read()
{
	int x=0;bool flag=1;char ch=getchar();
	while(ch<'0'||ch>'9') {if(ch=='-')flag=0;ch=getchar();}
	while(ch>='0'&&ch<='9') {x=(x<<1)+(x<<3)+ch-'0';ch=getchar();}
	if(flag) return x;
	return ~(x-1);
}
inline void write(int x)
{
	if(x<0) {x=~(x-1);putchar('-');}
	if(x>9) write(x/10);
	putchar(x%10+'0');
}
////////////////////////
ll n,k,d;
int main()
{
    freopen("sdoi2011_hbq.in","r",stdin);
	freopen("sdoi2011_hbq.out","w",stdout);
	cin>>n>>k>>d;
	cout<<(n-1)*(n-2)/2<<endl;
    return 0;
}