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/*#pragma GCC target ("avx2")
#pragma GCC optimize ("O3")
#pragma GCC optimize ("unroll-loops")*/

#include <bits/stdc++.h>
#define bit(n, i) (((n)>>(i))&1)
#define cat(x) cout << #x << " = " << x << "\n"
#define pb push_back
#define mp make_pair
#define se second
#define fi first
#define inf 2000000000
#define llinf 2000000000000000000LL
#define forn(i, a, b) for(ll i = (a); i <= (b); ++i)
#define all(x) (x).begin(), (x).end()
#define ppc(x) __builtin_popcount(x)
using namespace std;
typedef long long ll;
typedef double ld;
typedef pair<int, int> i2;
typedef vector<int> vi;
typedef vector<ll> vll;

const int N = 5e5+9;
bool vis[N];

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);

    int n, k; cin >> n >> k;
    int dcnt = 0;
    ll res = 0;
    bool ok = false;
    forn(i, 1, n) {
        int a; cin >> a;
        if(!vis[a]) {
            vis[a] = true;
            dcnt++;
            res += i-dcnt;
            if(dcnt == k) {
                ok = true;
                break;
            }
        }
    }
    cout << (ok ? res : -1);


    return 0;
}