#include<bits/stdc++.h>
#define LOCAL
#ifdef LOCAL
#define debug(x) cerr << #x << " = " << (x) << endl;
#define debugVec(v) cerr << #v << " = "; for (auto &e : v) cerr << e << " "; cerr << endl;
#define debugPair(p) cerr << #p << " = (" << p.first << ", " << p.second << ")" << endl;
#else
#define debug(x)
#define debugVec(v)
#define debugPair(p)
#endif
#define ll long long
#define pb push_back
#define mp make_pair
#define ff first
#define ss second
#define all(x) x.begin(),x.end()
#define rep(i,a,b) for(ll i=a;i<b;i++)
#define rrep(i,a,b) for(ll i=a;i>b;i--)
#define repi(i,a,b) for(ll i=a;i<=b;i++)
#define rrepi(i,a,b) for(ll i=a;i>=b;i--)
#define vi vector<int>
#define vll vector<ll>
#define vvi vector<vi>
#define ull unsigned ll
#define vvll vector<vll>
#define pii pair<int,int>
#define pll pair<ll,ll>
#define vpii vector<pii>
#define vpll vector<pll>
#define endl '\n'
#define IO ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define no cout << "NO" << endl;
#define yes cout << "YES" << endl;
using namespace std;
void solve(){
int n;
cin >> n;
vll vec(n);
rep(i , 0 , n) cin >> vec[i];
ll ans = 0 ;
int mlog = 0;
while((1 << (mlog + 1)) <= n){
mlog++;
}
rrepi(j , mlog , 0){
int len = 1 << j;
for(int l = 0 ;l + len <= n; l += len ){
ll mn = LLONG_MAX;
for(int i = l ; i < l + len;i++){
mn = min(mn , vec[i]);
}
if(mn > 0){
ans ++ ;
for(int i = l ;i < l + len ;i++){
vec[i]--;
}
}
}
}
rep(i , 0 , n){
if(vec[i] != 0) {
cout << -1 << endl;
return;
}
}
cout << ans << endl;
return;
}
int main() {
IO;
int t;
t = 1;
//cin >> t;
repi(i,1,t){
//cout<<"Case "<<i<<": ";
solve();
}
return 0;
}
