#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 7;
const int L = 22;
const int INF = 0x3f3f3f3f;
int n;
int a[N];
int dp[N][L];
int ndp[N][L];
int mn[N][L];
int kek[N];
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
cin >> n;
for (int i = 0; i < n; ++i) {
cin >> a[i];
}
for (int i = 0; i < n; ++i) {
if (a[i] >= L) {
cout << "-1\n";
return 0;
}
}
memset(dp, 0x3f, sizeof dp);
for (int i = 0; i < n; ++i) {
dp[i][a[i]] = 0;
if (a[i] > 0) {
dp[i][a[i] - 1] = 1;
}
}
for (int i = 0; i < n; ++i) {
mn[i][0] = dp[i][0];
}
for (int t = 1; (1 << t) <= n; ++t) {
memset(ndp, 0x3f, sizeof ndp);
for (int i = 0; i + (1 << t) <= n; ++i) {
for (int j = 0; j < L; ++j) {
ndp[i][j] = min(ndp[i][j], dp[i][j] + dp[i + (1 << (t - 1))][j]);
if (j + 1 < L) {
ndp[i][j] = min(ndp[i][j], dp[i][j + 1] + dp[i + (1 << (t - 1))][j + 1] + 1);
}
}
}
memcpy(dp, ndp, sizeof dp);
for (int i = 0; i + (1 << t) <= n; ++i) {
mn[i][t] = dp[i][0];
}
}
memset(kek, 0x3f, sizeof kek);
kek[n] = 0;
for (int i = n - 1; i >= 0; --i) {
for (int t = 0; i + (1 << t) <= n; ++t) {
kek[i] = min(kek[i], kek[i + (1 << t)] + mn[i][t]);
}
}
if (kek[0] == INF) {
cout << "-1\n";
return 0;
}
cout << kek[0] << endl;
return 0;
}
