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#include <iostream> 
#include <vector>
#include <string>
#include <cstring>
#include <algorithm>

using namespace std;
const int MAXN = 1000010;
int ex[MAXN] = {0};

int main() {
    ios::sync_with_stdio(false);

    int n, k, fst = 1;
    long long wn = 0;
    cin >> n >> k;

    for(int i = 1; i <= n; i++) { 
        int x;
        cin >> x;
        if (ex[x] == 0) {
            ex[x] = fst;
            wn += (i - fst);
            fst++;
        }
        if (fst > k) {
            break;
        }
    }
    if(fst <= k) {
        cout << -1 << endl;
    } else {
        cout << wn << endl;
    }
    return 0;
}