#include "bits/stdc++.h"
using namespace std;
#define ar array
const int N = 2e5 + 5;
struct ST{
int tree[N];
void add(int i, int v){ i++;
for(;i<N;i+=(i&-i)) tree[i] += v;
}
int get(int i){ i++;
int r=0;
for(;i>0;i-=(i&-i)) r += tree[i];
return r;
}
}tree;
signed main(){
ios::sync_with_stdio(0); cin.tie(0);
int n; cin>>n;
vector<int> a(n);
vector<vector<int>> to(n);
for(int i=0;i<n;i++){
cin>>a[i];
to[--a[i]].push_back(i);
}
priority_queue<int> q;
long long res = 0;
for(int i=n-1;~i;i--){
for(auto x : to[i]) q.push(x);
if(q.empty()){
cout<<-1<<"\n";
return 0;
}
int p = q.top(); q.pop();
int j = p + tree.get(p);
res += (i - j);
tree.add(p, -1);
}
cout<<res<<"\n";
}
