#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <queue>
typedef long long ll;
using namespace std;
const int maxn = 1 << 18;
int maxi[maxn * 2], alive[maxn * 2];
void merge(int i)
{
maxi[i] = max(maxi[i << 1], maxi[i << 1 | 1]);
alive[i] = alive[i << 1] + alive[i << 1 | 1];
}
void remove_element(int i)
{
i += maxn;
maxi[i] = -1, alive[i] = 0;
for (i = (i >> 1); i > 0; i >>= 1) merge(i);
}
int last_big(int x)
{
int i = 1;
for (; i < maxn; )
{
if (maxi[i << 1 | 1] < x) i = i << 1;
else i = i << 1 | 1;
}
return i - maxn;
}
int sum(int l, int r)
{
int s = 0;
for (l += maxn, r += maxn + 1; l < r; l >>= 1, r >>= 1)
{
if (l & 1) s += alive[l++];
if (r & 1) s += alive[--r];
}
return s;
}
int main()
{
ios::sync_with_stdio(false);
cin.tie(0);
for (int i = 0; i < maxn * 2; i++) maxi[i] = -1;
int n;
cin >> n;
vector<int> d(n);
for (int i = 0; i < n; i++) cin >> d[i], maxi[i + maxn] = --d[i], alive[i + maxn] = 1;
for (int i = maxn - 1; i > 0; i--) merge(i);
vector<int> v = d;
sort(v.begin(), v.end());
for (int i = 0; i < n; i++) if (v[i] < i)
{
cout << "-1\n";
return 0;
}
ll ans = 0;
for (int i = n - 1; i >= 0; i--)
{
int j = last_big(i);
//cout << j << endl;
ans += sum(j, n - 1) - 1;
remove_element(j);
}
cout << ans << "\n";
return 0;
}
