#include<stdio.h>
#include<set>
using namespace std;
set<int> s;
set<int>::iterator it;
int v[5001];
int main(){
    freopen("secv.in","r",stdin);
    freopen("secv.out","w",stdout);
    int n,i,a,b;
    scanf("%d",&n);
    for(i=1;i<=n;i++){
        scanf("%d",&v[i]);
        s.insert(v[i]);
    }
    it=s.begin();
    i=1;
    while(v[i]!=*it)
        i++;
    a=i;
    it++;
    i++;
    while(i<=n&&it!=s.end()){
        if(v[i]==*it)
            it++;
        i++;
    }
    if(it!=s.end())
        printf("-1");
    else
        printf("%d",i-a);
    return 0;
}