/*http://www.infoarena.ro/problema/secv*/
#include <fstream>
#include <set>
#define N 5001
using namespace std;
set<int> val;
int v[N], st[N], sol[N], n, i, nr_dist;

int dinamica()
{
    int scmax = 1, i, j, poz;

    for(i=1; i<n; ++i)
    {
        for(j=i-1; j>=0; --j)
            if(v[j] < v[i] && sol[i] < sol[j]+1)
            {
                sol[i] = sol[j]+1;
                st[i] = st[j];
            }
        if(sol[i] > scmax)
        {
            scmax = sol[i];
            poz = i;
        }
    }
    ++scmax;
    if(scmax < nr_dist) return -1;
    return poz - st[poz] + 1;
}

int main()
{
    ifstream f("secv.in");
    ofstream g("secv.out");
    f >> n;
    for(i=0; i<n; ++i)
    {
        f >> v[i];
        if(val.find(v[i]) == val.end())
        {
            ++nr_dist;
            val.insert(v[i]);
        }
        st[i] = i;
    }
    g << dinamica();
    f.close();
    g.close();
    return 0;
}