#include <bits/stdc++.h>

using namespace std;
mt19937 rnd ( time ( 0 ) );

#define int long long
#define fir first
#define sec second
#define mkp make_pair 
#define pb push_back
#define lep( i, l, r ) for ( int i = ( l ); i <= ( r ); ++ i )
#define rep( i, r, l ) for ( int i = ( r ); i >= ( l ); -- i )

typedef long long ll;
typedef long double ld;
typedef pair < int, int > pii;

char _c; bool _f; template < class type > inline void read ( type &x ) {
	_f = 0, x = 0;
	while ( _c = getchar (), !isdigit ( _c ) ) if ( _c == '-' ) _f = 1;
	while ( isdigit ( _c ) ) x = x * 10 + _c - '0', _c = getchar (); if ( _f ) { x = -x; }
}

template < class type > inline void chkmin ( type &x, type y ) { x = ( x <= y ? x : y ); }
template < class type > inline void chkmax ( type &x, type y ) { x = ( x >= y ? x : y ); }

const int N = 1e6 + 5;

int n, m, p, ans;
int a[N], f[11][N], suf[11][N], pre[N];

void Solve () {
  cin >> n >> m >> p;
  for ( int i = 1; i <= n; i ++ ) {
    cin >> a[i];
    pre[i] = pre[i - 1] + a[i];
  }
  memset ( suf, 0xcf, sizeof ( suf ) );
  for ( int i = 1; i <= m; i ++ ) {
    for ( int j = i; j <= n; j ++ ) {
      f[i][j] = p * ceil ( ( j - i + 1 ) * 1.0 / m );
    }
    for ( int j = n; j >= i; j -- ) {
      suf[i][j] = max ( suf[i][j + 1], pre[j] - f[i][j] );
    }
  }
  for ( int k = 1; k <= m; k ++ ) {
    for ( int i = k, j = 0; i <= n; i += m, j ++ ) {
      ans = max ( ans, j * p + suf[k][i] - pre[i - 1] );
    }
  }
  cout << ans;
}

signed main () {
#ifdef judge
  freopen ( "Code.in", "r", stdin );
  freopen ( "Code.out", "w", stdout );
  freopen ( "Code.err", "w", stderr );
#endif
  Solve ();
  return 0;
}