#include<bits/stdc++.h>
#define int long long
#define db double
using namespace std;
const int Max=1e6+5;
int a[Max],sum[Max];
int n,m,k;
int ans=0;
int l[105];
int calc(int p,int pos)
{
	return sum[p-1]+k*((int)ceil((db)(pos-p+1)/m));
}
signed main()
{
	//freopen("A.in","r",stdin);
	//freopen("A.out","w",stdout);
	
	scanf("%lld%lld%lld",&n,&m,&k);
	for(int i=1;i<=n;i++)
	{
		scanf("%lld",&a[i]);
		sum[i]=sum[i-1]+a[i];
	}
	for(int i=1;i<=n;i++)
	{
		int pos=i%m;
		if(l[pos]==0 or calc(l[pos],i)>=calc(i,i))
			l[pos]=i;
		for(int j=0;j<m;j++)
		{
			if(l[j]==0)continue;
			ans=max(ans,sum[i]-calc(l[j],i));
		}
	}
	printf("%lld",ans);
}