只得了42分，$k=1$ 的情况全过了，但是 $k=2$ 的情况有几个点WA，求查错

#include <cstdio>
#include <algorithm>
#include <functional>
using namespace std;
#define impossible {puts("-1");return 0;}
#define ll long long
const int N=3e5+5;
int n,m,k;
ll a[N];
ll f(int l,int r){
	ll kase=m-1;
	ll ans=0;int i;
	for(i=1;i<=m;++i){
		if(kase>0){
			ans+=kase*a[l++];
		}
		else if(kase<0){
			break;
		}
		kase-=2;
	}
	kase=-m+1;
	while(kase<0){
		ans+=kase*a[r--];
		kase+=2;
	}
	return ans;
}
int main()
{
	scanf("%d%d%d",&n,&m,&k);
	for(int i=1;i<=n;++i){
		scanf("%lld",&a[i]);
	}
	if(k==2&&n==m) impossible;
	sort(a+1,a+n+1,greater<ll>());
	ll bans=f(1,n);
	if(k==1){
		printf("%lld\n",bans);
	}
	else{
		ll ans1=f(2,n),ans2=f(1,n-1),ans=0;
		if(ans1<bans) ans=max(ans,ans1);
		if(ans2<bans) ans=max(ans,ans2);
		printf("%lld\n",ans);
	}
	return 0;
}