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#include <bits/stdc++.h>

using namespace std;

#define FOR(i,a,b) for(int i = (a); i <= (b); ++i)
#define FORD(i,a,b) for(int i = (a); i >= (b); --i)
#define RI(i,n) FOR(i,1,(n))
#define REP(i,n) FOR(i,0,(n)-1)
#define mini(a,b) a=min(a,b)
#define maxi(a,b) a=max(a,b)
#define pb push_back
#define st first
#define nd second
#define sz(w) (int) w.size()
typedef vector<int> vi;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<pii, int> para;
const ll inf = 1e18 + 7;
const ll maxN = 1e6 + 5;
const ll MOD = 1e9 + 7;

// sprawdz MODULO!
int main() {
    mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
	ios_base::sync_with_stdio(0);
	cin.tie(0); cout.tie(0);
    int n, k; cin >> n >> k;
    vi a(n), frq(n + 1);
    REP(i, n) cin >> a[i];
    int last = -1, diff = 0;
    REP(i, n) {
        if (frq[a[i]] == 0) {
            diff++;
            if (diff == k) {
                last = i; break;
            }
        }
        frq[a[i]]++;
    }
    if (last == -1) {
        cout << -1 << endl;
        return 0;
    }
    ll ans = 0;
    FORD(i, last, 0) {
        frq[a[i]]--;
        if (frq[a[i]] > 0) {
            ans += (last - i);
            last--;
        }
    }
    cout << ans << endl;
	return 0;
}