#include<bits/stdc++.h>
using namespace std;
#define task "a"
#define se second
#define fi first
#define ll long long
#define ii pair<ll, ll>
const long mxN = 2e5 + 7;
int n, a[mxN];
namespace Segment
{
struct node
{
int cnt, mx;
} tree[mxN * 4];
node Merge(node u, node v)
{
u.cnt += v.cnt;
u.mx = max(u.mx, v.mx);
return u;
}
void Build(int j = 1, int l = 1, int r = n)
{
if (l == r)
{
tree[j].cnt = 1;
tree[j].mx = a[l];
return;
}
int mid = (l + r) / 2;
Build(j * 2, l, mid);
Build(j * 2 + 1, mid + 1, r);
tree[j] = Merge(tree[j * 2], tree[j * 2 + 1]);
}
int Get(int vt, int j = 1, int l = 1, int r = n)
{
if (l == r)
{
tree[j].cnt = 0;
tree[j].mx = 0;
return 0;
}
int mid = (l + r) / 2;
int res = 0;
if (tree[j * 2 + 1].mx >= vt)
res = Get(vt, j * 2 + 1, mid + 1, r);
else
res = tree[j * 2 + 1].cnt + Get(vt, j * 2, l, mid);
tree[j] = Merge(tree[j * 2], tree[j * 2 + 1]);
return res;
}
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
//freopen(task".INP", "r", stdin);
//freopen(task".OUT", "w", stdout);
cin >> n;
for (int i = 1; i <= n; i++)
cin >> a[i];
Segment::Build();
ll ans = 0;
for (int i = n; i >= 1; i--)
{
if (Segment::tree[1].mx < i)
{
cout << -1;
return 0;
}
ans += Segment::Get(i);
}
cout << ans;
}
