#include<iostream>
#include <bits/stdc++.h>

#define ll long long
#define ld long double
#define IO ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
using namespace std;
const int N = 100 + 5, mod = 1e9 + 7;

void doWork() {
    int n, l, r;
    cin >> n >> l >> r;
    vector<ll> x(n);
    for (int i = 0; i < n; i++) {
        cin >> x[i];
    }
    sort(x.begin(), x.end());
    for (int i = 1; i < n; i++) {
        if (x[i] - x[i - 1] < l) {
            cout << -1;
            return;
        }
    }
    ll ans = 0;
    long double last = -2e9;
    x.push_back(3e9);
    for (int i = 0; i < n; i++) {
        long double mn = min(x[i + 1] - (ld) ((ld)l / 2) - x[i], x[i] - last);
        mn = min(mn, (ld) r / 2);
        ans += round(mn * 2);
        last = x[i] + mn;
    }
    cout << ans;
}

int main() {
    IO
    int t = 1;
    //cin >> t;
    for (int i = 1; i <= t; i++) {
        //  cout << "Case #" << i << ": ";
        doWork();
    }
}