#include <iostream>
#include <vector>
#include <algorithm>
using namespace std ;
using ll = long long ;
int main() {
   ll n, small, large ;
   cin >> n >> small >> large ;
   ll u = large - small, v = small * n ;
   vector<ll> p(n) ;
   for (auto &vv: p)
      cin >> vv ;
   sort(p.begin(), p.end()) ;
   for (int i=1; i<n; i++) {
      ll g = 2 * (p[i] - p[i-1] - small) ;
      if (g < 0) {
         cout << -1 << endl ;
         exit(0) ;
      }
      v += min(u, g) ;
      u = min(large - small, max(0LL, g - u)) ;
   }
   cout << u+v << endl ;
}