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#include<bits/stdc++.h>

#define st first
#define nd second
#define pb(x) push_back(x)
#define pp(x) pop_back(x)
#define mp(a, b) make_pair(a, b)
#define all(x) (x).begin(), (x).end()
#define rev(x) reverse(all(x))
#define sor(x) sort(all(x))
#define sz(x) (int)(x).size()
#define rsz(x) resize(x)

using namespace std;

typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<pii > vii;
typedef vector<ll> vl;
typedef vector<pll> vll;
typedef string str;
#define BOOST ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
const int MN=5*(int)1e5+5;
bitset <MN> tab;
int main()
{
    BOOST
    int n,k,buf;
    cin>>n>>k;
    queue <int> powt; queue <int> nowe;
    for(int i=0; i<n; i++)
    {
        cin>>buf;
        if(tab[buf]!=0 and i<k) powt.push(i);
        if(tab[buf]==0 and i>=k) nowe.push(i);
        tab[buf]=1;
    }
    int sum=tab.count();
    if(sum<k)
    {
        cout<<-1; return 0;
    }
    ll wyn=0;
    while(powt.empty()==0)
    {
        wyn+=(nowe.front()-powt.front());
        nowe.pop(); powt.pop();
    }
    cout<<wyn;
    return 0;
}