#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
struct SegTree {
int n;
vector<ll> mn, lz;
SegTree(int _n, const vector<ll>& a) : n(_n) {
mn.assign(4*n, 0);
lz.assign(4*n, 0);
build(1, 0, n-1, a);
}
void build(int node, int l, int r, const vector<ll>& a) {
if (l == r) {
mn[node] = a[l];
} else {
int mid = (l + r) >> 1;
build(node<<1, l, mid, a);
build(node<<1|1, mid+1, r, a);
mn[node] = min(mn[node<<1], mn[node<<1|1]);
}
}
void apply(int node, ll val) {
mn[node] += val;
lz[node] += val;
}
void push(int node) {
if (lz[node] != 0) {
apply(node<<1, lz[node]);
apply(node<<1|1, lz[node]);
lz[node] = 0;
}
}
ll queryMin(int node, int l, int r, int ql, int qr) {
if (qr < l || r < ql) return LLONG_MAX;
if (ql <= l && r <= qr) return mn[node];
push(node);
int mid = (l + r) >> 1;
return min(queryMin(node<<1, l, mid, ql, qr),
queryMin(node<<1|1, mid+1, r, ql, qr));
}
void update(int node, int l, int r, int ql, int qr, ll val) {
if (qr < l || r < ql) return;
if (ql <= l && r <= qr) {
apply(node, val);
return;
}
push(node);
int mid = (l + r) >> 1;
update(node<<1, l, mid, ql, qr, val);
update(node<<1|1, mid+1, r, ql, qr, val);
mn[node] = min(mn[node<<1], mn[node<<1|1]);
}
};
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n;
cin >> n;
vector<ll> a(n);
for (int i = 0; i < n; i++) cin >> a[i];
SegTree st(n, a);
ll moves = 0;
int maxk = 0;
while ((1 << (maxk + 1)) <= n) maxk++;
// Greedy: largest segments first
for (int k = maxk; k >= 0; --k) {
int len = 1 << k;
for (int l = 0; l + len <= n; l += len) {
int r = l + len - 1;
if (st.queryMin(1, 0, n-1, l, r) > 0) {
st.update(1, 0, n-1, l, r, -1);
moves++;
}
}
}
// Check if any element remains positive -> impossible
for (int i = 0; i < n; ++i) {
if (st.queryMin(1, 0, n-1, i, i) > 0) {
cout << -1;
return 0;
}
}
cout << moves;
return 0;
}
