#include <bits/stdc++.h>
using namespace std;
string z = "secv.";

ifstream f(z+"in");
ofstream g(z+"out");

int i,j,n,st,v[5001],w[5001],sol=1<<30;

int main()
{
f>>n;
for(i=0;i<n;++i)
    f>>v[i],w[i]=++v[i];
sort(w,w+n);
w[unique(w,w+n)-w]=0;
for (j=n-1;j>=0;--j)
if(w[0]==v[j])
{
    st=0;
    for (i=j;i<n;++i)
    {
        if (v[i]==w[st])
            ++st;
        if (!w[st])
            sol=min(sol,i-j+1);
    }
}
if (sol!=1<<30)g<<sol;
else g<<-1;

return 0;
}