#include <bits/stdc++.h>
using namespace std;
const int maxn = 200010;
int n, d[maxn], p[maxn];
#define mid ((l + r) >> 1)
#define ls (k << 1)
#define rs (k << 1 | 1)
int mx[maxn << 2];
void build(int k, int l, int r) {
if (l == r) { mx[k] = d[l]; return; }
build(ls, l, mid), build(rs, mid + 1, r);
mx[k] = max(mx[ls], mx[rs]);
}
void upd(int k, int l, int r, int p) {
if (l == r) { mx[k] = 0; return; }
mid >= p ? upd(ls, l, mid, p) : upd(rs, mid + 1, r, p);
mx[k] = max(mx[ls], mx[rs]);
}
int find(int k, int l, int r, int v) {
if (l == r) return l;
return mx[rs] >= v ? find(rs, mid + 1, r, v) : find(ls, l, mid, v);
}
namespace BIT {
int c[maxn];
void add(int p, int v) {
for (; p <= n; p += p & -p) c[p] += v;
}
int sum(int p) {
int s = 0;
for (; p; p -= p & -p) s += c[p];
return s;
}
} // namespace BIT
int main() {
ios::sync_with_stdio(0), cin.tie(0);
cin >> n;
for (int i = 1; i <= n; i++) {
cin >> d[i];
}
build(1, 1, n);
for (int i = n; i; i--) {
if (mx[1] < i) puts("-1"), exit(0);
int j = find(1, 1, n, i);
p[j] = i, upd(1, 1, n, j);
}
long long ans = 0;
for (int i = n; i; i--) {
ans += BIT::sum(p[i]), BIT::add(p[i], 1);
}
cout << ans << "\n";
return 0;
}
