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#include <bits/stdc++.h>

#pragma GCC optimize(-O3)
#pragma GCC optimize(Ofast)
#pragma GCC optimize("unroll-loops")

#define fi first
#define se second
#define p_b push_back
#define pll pair<ll,ll>
#define pii pair<int,int>
#define m_p make_pair
#define all(x) x.begin(),x.end()
#define sset ordered_set
#define sqr(x) (x)*(x)
#define pw(x) (1ll << (x))
#define sz(x) (int)x.size()
#define fout(x) {cout << x << "\n"; return; }

using namespace std;
typedef long long ll;
typedef long double ld;
const int MAXN = 1e5;
const int M = pw(16);
const long long mod = 998244353;
const int N = 2e5;
const int inf = 1e9;

template <typename T> void vout(T s){cout << s << endl;exit(0);}

int main(){

    ios_base::sync_with_stdio(0);
    cin.tie(0);

    int n, k;
    cin >> n >> k;

    map <int, int> mp;
    long long ans = 0, s = 0;

    vector <int> a(n);

    for(auto &i : a) cin >> i;

    int uk = 0;

    for(int i = 1; i <= k; i++){        
        while(true) {
            while(uk < n && mp[a[uk]]){
                s++;
                uk++;
            }
            if(uk == n) vout(-1);            
            mp[a[uk]] = 1;
            ans += s;
            uk++;
            break;
        }
    }

    cout << ans << '\n';

    return 0;
}