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#include<bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

#define ST first
#define ND second
#define SIZE(a) ((int)a.size())


int main() {
    ios_base::sync_with_stdio(0);
    int n, k;
    cin >> n >> k;
    vector<bool> visited(n);
    int total = 0;
    ll res = 0;
    for(int i=0; i < n; i++) {
        int a;
        cin >> a;
        a--;
        if(total < k && !visited[a]) {
            visited[a] = true;
            res += i-total;
            total +=1;
        }
    }
    if(total < k) {
        cout << "-1\n";
    } else {
        cout << res << "\n";
    }
}