#include<bits/stdc++.h>
#define ll long long
#define st first
#define nd second
using namespace std;
const int N = 3e5+1;
int n;
ll m, k, ans;
int main() {
    ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    cin>>n>>m>>k;
    vector<int> a(n);
    vector<ll> pref(n);
    for(int i=0; i<n; ++i) {
        cin>>a[i];
        pref[i]=a[i];
        if(i!=0) pref[i]+=pref[i-1];
    }
    for(int mod=0; mod<m; ++mod) {
        vector<pair<ll, int>> b(n);
        for(int i=mod; i<n; ++i) {
            b[i].st=pref[i];
            if(mod!=0) b[i].st-=pref[mod-1];
            b[i].st-=k*((ll)(i-mod)/m+(ll)1);
            b[i].nd=i;
        }
        sort(b.begin(), b.end());
        reverse(b.begin(), b.end());
        int j = 0;
        for(int i=0; mod+i*m<n; i++) {
            while(j<n-1 && b[j].nd<mod+i*m) {
                j++;
            }
            ans = max(ans, b[j].st+(ll)i*k-( ((mod+i*m==0)? 0 : pref[mod+i*m-1]) - ((mod==0)? 0 : pref[mod-1]) ));
        }
    }
    cout<<ans;
}