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#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using ll   = long long;
using ull  = unsigned long long;
using ld   = long double;
using pii  = pair<int,int>;
using pll  = pair<ll,ll>;
using vi   = vector<int>;
using vb   = vector<bool>;
using vc   = vector<char>;
using vvc  = vector<vc>;
using vvi  = vector<vi>;
using vvb  = vector<vb>;
using vpii = vector<pii>;
using vpll = vector<pll>;
using vll  = vector<ll>;
using vull = vector<ull>;
using vvll = vector<vll>;
using namespace __gnu_pbds;
template <typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
#define f first
#define s second
#define pb emplace_back
#define rep(i, begin, end) for(auto i = (begin); i <= (end); ++i)
#define repr(i, begin, end) for(auto i = (begin); i >= (end); --i)
#define bend(X) X.begin(), X.end()
#ifdef LOCAL
#define deb(...) cout << #__VA_ARGS__ << " = " << __VA_ARGS__ << endl
#define say(...) cout << __VA_ARGS__ << endl
#else
#define deb(x)
#define say(x)
#define endl '\n'
#endif
constexpr int INF = 1e9+1e7;
constexpr ll INFl = INF;
constexpr ll INFll = 1e18+1e16;
void print() { cout << '\n'; }
template <typename T, typename... Args> void print(T x, Args... args) { cout << x << ' '; print(args...); }
template <typename T1, typename T2> ostream& operator<<(ostream& os, pair<T1,T2> x) { os << x.f << ' ' << x.s; return os; }
template <typename T1, typename T2> istream& operator>>(istream& is, pair<T1,T2>& x) { is >> x.f >> x.s; return is; }
template <typename Ch, typename Tr, typename T> ostream& operator<<(basic_ostream<Ch,Tr>& os, T x) { for(auto i : x) os << i << ' '; return os; }
template <typename T> istream& operator>>(istream& is, vector<T>& x) { for(T& i : x) is >> i; return is; }
template <typename T1, typename T2> pair<T1,T2> operator+(const pair<T1,T2>& A, const pair<T1,T2>& B) { return {A.f+B.f, A.s+B.s}; }
template <typename T1, typename T2> pair<T1,T2> operator-(const pair<T1,T2>& A, const pair<T1,T2>& B) { return {A.f-B.f, A.s-B.s}; }
template <typename T1, typename T2> pair<T1,T2>& operator+=(pair<T1,T2>& A, const pair<T1,T2>& B) { A.f += B.f; A.s += B.s; return A; }
template <typename T1, typename T2> pair<T1,T2>& operator-=(pair<T1,T2>& A, const pair<T1,T2>& B) { A.f -= B.f; A.s -= B.s; return A; }
template <typename T1, typename T2> pair<T1,T2> operator-(const pair<T1,T2>& A) { return {-A.f, -A.s}; }
template <typename T1, typename T2> bool maxe(T1& A, const T2& B) { if(B>A) { A=B; return 1; } return 0; }
template <typename T1, typename T2> bool mine(T1& A, const T2& B) { if(B<A) { A=B; return 1; } return 0; }
class Nic{} nic;
template <typename T> Nic operator<<(Nic, T) { return nic; }
Nic operator<<(ostream&, Nic) { return nic; }

//////////////////////////////////////////////////

int n, k;
vi T;
vb vis;

auto solve() {
    cin >> n >> k;
    T.resize(n);
    cin >> T;
    vis.resize(n+1);
    ll R = 0;
    int i = 0;
    int d = 0;
    rep (l, 1, k) {
        if (i == n) return -1ll;
        while (vis[T[i]]) {
            i++;
            d++;
            if (i == n) return -1ll;
        }
        vis[T[i]] = 1;
        R += d;
        i++;
    }

    return R;
}



int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int noOfTests = 1;
    //cin >> noOfTests;

    while(noOfTests --> 0)
        cout << solve() << '\n';

    return 0;
}