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#include <bits/stdc++.h>
#define dbg(x) " [" << #x << ": " << (x) << "] "
using namespace std;
template<typename A,typename B>
ostream& operator<<(ostream& out,const pair<A,B>& p) {
    return out << "(" << p.first << ", " << p.second << ")";
}
template<typename T>
ostream& operator<<(ostream& out,const vector<T>& c) {
    out << "{";
    for(auto it = c.begin(); it != c.end(); it++) {
        if(it != c.begin()) out << ", ";
        out << *it;
    }
    return out << "}";
}
void solve() {
    int n,k;
    cin >> n >> k;
    vector<int> v(n);
    for(int& i : v) cin >> i;
    vector<int> u(n + 1);
    int j = 0;
    long long ans = 0;
    for(int i = 0; i < n; i++) {
        if(!u[v[i]] && j < k) {
            ans += i - j;
            j++;
        }
        u[v[i]] = 1;
    }
    if(j < k) {
        cout << -1 << endl;
    } else {
        cout << ans << endl;
    }
}
int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int testNum = 1;
    //cin >> testNum;
    for(int i = 1; i <= testNum; i++) {
        solve();
    }
    return 0;
}