#include <bits/stdc++.h>
#include <bits/extc++.h>
using namespace __gnu_pbds;
using namespace std;
#define ll long long
#define pll pair<ll,ll>
#define pii pair<int,int>
#define fs first
#define sc second
#define tlll tuple<ll,ll,ll>
#define ordered_set(T) tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>
ordered_set(int) st,pos;
const int mxn = 2e5+10;
int arr[mxn];
std::priority_queue<pii,vector<pii>,less<pii>> pq;
int N;
int main(){
ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
cin>>N;
for(int i = 1;i<=N;i++)cin>>arr[i],pos.insert(i),pq.push(make_pair(arr[i],i));
ll ans = 0;
while(!pos.empty()){
int now = pos.size();
while(!pq.empty()&&pq.top().fs>=now){
if(pos.find(pq.top().sc) != pos.end()){
st.insert(pq.top().sc);
pq.pop();
}
}
/*
cout<<now<<":"<<endl;
for(auto &i:st)cout<<i<<' ';cout<<endl;
for(auto &i:pos)cout<<i<<' ';cout<<endl;
*/
if(st.empty()){
cout<<-1;
return 0;
}
int tar = *st.rbegin();
ans += now-pos.order_of_key(tar)-1;
pos.erase(tar);
st.erase(tar);
}
cout<<ans;
}
