//条件为一个数+1一个数-1，即总和不变，先判断总和再排序后依次取各个数差的绝对值除2 #include<iostream>

#include<cmath>

#include<algorithm>

using namespace std;

#define ll long long

int main()

{

ios::sync_with_stdio(0);

cin.tie(0);

cout.tie(0);

ll n,sum1=0;

cin>>n;

ll a[n+10];

for(int i=1;i<=n;i++){

cin>>a[i];

sum1+=a[i];

}

ll sum2=(n+n*n)/2;

if(sum1!=sum2){

cout<<-1<<"\n";

}else{

ll ans=0;

sort(a+1,a+n+1);

for(int i=1;i<=n;i++){

if(a[i]!=i){

ans=ans+abs(a[i]-i);

}

}

cout<<ans/2<<"\n";

}

return 0;

}