#include <bits/stdc++.h>
#define f first
#define s second
#define pb push_back
#define ll long long
using namespace std;
int a[200005],b[200005];
int main(){
ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
int n;
cin>>n;
for (int i = 1; i <= n; i++)
cin>>a[i],b[i] = a[i];
sort(b + 1,b + n + 1);
for (int i = 1;i <= n; i++)
if (b[i] < i) {
cout<<-1;
return 0;
}
int ans=0;
while (true){
int ind = 0;
for (int i = 1; i <= n; i++)
if (a[i] < i) {ind = i; break;}
if (ind == 0) break;
//cout<<ind<<" "<<a[ind]<<endl;
int m = a[ind];
for (int i = ind - 1; i >= 1; i--){
if (a[i] == a[i + 1]) continue;
swap(a[i],a[i + 1]);
ans++;
if (m >= i) break;
}
/*
for (int i =1;i<=n;i++)
cout<<a[i]<<" ";
cout<<endl;
*/
}
cout<<ans;
}
