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#include <bits/stdc++.h>
using namespace std;
using LL = long long;
#define FOR(i, l, r) for(int i = (l); i <= (r); ++i)
#define REP(i, n) FOR(i, 0, (n) - 1)
#define ssize(x) int(x.size())
template<class A, class B> auto& operator<<(ostream &o, pair<A, B> p) {
    return o << '(' << p.first << ", " << p.second << ')';
}
template<class T> auto operator<<(ostream &o, T x) -> decltype(x.end(), o) {
    o << '{'; int i = 0; for(auto e : x) o << (", ")+2*!i++ << e; return o << '}';
}
ostream& operator<<(ostream &o, string &s) {
    return o << s.data();   
}
#ifdef DEBUG
#define debug(x...) cerr << "[" #x "]: ", [](auto... $) {((cerr << $ << "; "), ...); }(x), cerr << '\n'
#else
#define debug(...) {}
#endif

int main() {
    cin.tie(0)->sync_with_stdio(0);

    int n, k;
    cin >> n >> k;
    vector<int> a(n);
    REP(i, n) {
        cin >> a[i];
        a[i]--;
    }
    
    LL ans = 0;
    vector<bool> in(n);
    int dif = 0, idx = 0;
    REP(i, n) {
  		int x = a[i];
    	if (!in[x]) {
    		in[x] = true;
    		dif++;
    		ans += (LL) (i - idx);
    		idx++;
    		if (dif == k)
    			break;
    	}
    }

    if (dif != k)
    	ans = -1;
    cout << ans << '\n';

    return 0;
}