#include <bits/stdc++.h>
using namespace std;
#define sonic ios_base::sync_with_stdio(false);cin.tie(0); cout.tie(0)
#define IO(main) if(fopen(main".inp","r")){freopen(main".inp","r",stdin);freopen(main".out","w",stdout);}
#define pb push_back
#define fi first
#define se second
#define mp make_pair
#define ins insert
#define pb push_back
#define el cout << endl
#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(), (x).end()
#define MASK(i) ((1LL)<<(i))
#define BIT(x,i) (((x)>>(i))&(1LL))
#define FOR(i, a, b) for(int (i)=(a);(i)<=(b); i++)
#define FORD(i, a, b) for(int (i)=(a);(i)>=(b); i--)
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vi = vector<int>;
using vii = vector<pii>;
const int N = 1e6 + 9;
const int mod = 1e9 + 7;
const ll INF = 1e18 + 7;
const int base = 31;
const int LOG = 20;
const ll MOD = 1e9 + 7;
int mul(int x, int y) {return 1LL * x * y % mod;}
int calPw(int x, int y)
{
int ans = 1;
while(y)
{
if (y&1) ans = 1LL * ans * x % mod;
x = 1LL * x * x % mod;
y >>= 1;
}
return ans;
}
int d4x[4] = {1, 0, -1, 0};
int d4y[4] = {0, 1, 0, -1};
int dx[8] = {0, 1, 1, 1, 0, -1, -1, -1};
int dy[8] = {1, 1, 0, -1, -1, -1, 0, 1};
///Author: Le Chi Thien Luong The Vinh high school 2008
///code
template<class X, class Y>
bool minimize(X &x , Y y){
if(x > y){
x = y;
return true;
}
return false;
}
template<class X, class Y>
bool maximize(X &x , Y y){
if(x < y){
x = y;
return true;
}
return false;
}
template<class X, class Y>
bool add(X &x , Y y){
if((x + y ) >= mod){
ll tmp = x + y;
x = tmp - mod;
return true;
}
return false;
}
void Solve(){
}
int n, a[N], pos[N];
set<int>v;
ll bit[N];
void update(int u, int val){
while(u <= n){
bit[u] += val;
u += (u&-u);
}
}
ll get(int u){
int res = 0;
while(u > 0){
res +=bit[u];
u -= (u&-u);
}
return res;
}
void Read(){
cin >> n;
FOR(i, 1, n){
cin >> a[i];
v.insert(i);
}
FORD(i, n, 1){
auto it = v.upper_bound(a[i]);
if(it == v.begin()){
cout << -1;
return;
}
it--;
pos[i] = *it;
v.erase(it);
}
ll ans = 0;;
FORD(i, n, 1){
ans += get(pos[i]);
update(pos[i], 1);
}
cout << ans;
}
int main()
{
sonic;
int TEST = 1;
// cin >> TEST;
while(TEST--)
{
Read();
Solve();
}
}
