#include<bits/stdc++.h>
using namespace std;
#define inf 0x3f3f3f3f
#define sz(x) int((x).size())
#define fi first
#define se second
typedef long long ll;
typedef pair<int, int> ii;
template<class X, class Y>
inline bool maximize(X &x, const Y &y) {return (x < y ? x = y, 1 : 0);}
template<class X, class Y>
inline bool minimize(X &x, const Y &y) {return (x > y ? x = y, 1 : 0);}
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int Random(int l, int r) {
return uniform_int_distribution<int>(l, r)(rng);
}
const int MAXN = 1e7+7;
int a[MAXN], nArr, k, L;
#ifdef Nhoksocqt1
int delivery(int _N, int _K, int _L, int _pos[]);
int main() {
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
freopen("BOXES.inp", "r", stdin);
freopen("BOXES.out", "w", stdout);
process();
return 0;
}
#endif // Nhoksocqt1
ll delivery(int _N, int _K, int _L, int _pos[]) {
nArr = _N, k = _K, L = _L;
for (int i = 1; i <= nArr; ++i)
a[i] = _pos[i - 1];
ll result(0);
int l(1), r(nArr);
while(l <= r) {
int costLeft = min(_L, 2 * a[min(r, l + _K - 1)]);
int costRight = min(_L, 2 * (_L - a[max(l, r - _K + 1)]));
result += min(costLeft, costRight);
if(costLeft <= costRight) {
l += _K;
} else {
r -= _K;
}
}
return result;
}
