#include <bits/stdc++.h>
using namespace std;

#define ll long long

void solve(){
    ll n, s, k;
    cin >> n >> s >> k;
    if(n == 1){
        cout << k;
        return;
    }

    s = 2*s;
    k = 2*k;

    vector<ll> x(n);
    for(ll i = 0; i < n; i++){
        cin >> x[i];
        x[i] = 2*x[i];
    }

    sort(x.begin(), x.end());
    
    vector<ll> ans(n, s);
    ans[0] = min(2*(x[1] - x[0] - s/2), k); 
    bool ok = true;
    for(ll i = 1; i < n; i++){
        if(x[i] - x[i-1] < s){
            ok = false;
            break;
        }

        if(i < n-1) ans[i] = min(k, min(2*(x[i] - x[i-1] - ans[i-1]/2), 2*(x[i+1] - x[i] - s/2)));
        else ans[i] = min(2*(x[i] - x[i-1] - ans[i-1]/2), k);
    }

    // for(ll i = 0; i < n; i++){
    //     cout << ans[i] << " ";
    // }

    ll val = 0;
    for(ll i = 0; i < n; i++){
        val += ans[i];
    }

    val = val/2;

    if(!ok){
        cout << -1;
    }
    else{
        cout << val;
    }
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int t = 1;
    //cin >> t;
    while(t--){
        cout << fixed << setprecision(15);
        solve();
    }

    return 0;
}