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#include <iostream>
using namespace std;

int n, k;
int but[1000000];
bool is_at_start[1000000];
int at_start;
long long res, final_res;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cin >> n >> k;
    at_start = 0;
    res = 0;
    final_res = -1;

    for(int i = 0; i < n; i++) {
        cin>>but[i];
        is_at_start[i+1] = false;
    }

    for(int i = 0; i < n && at_start < k; i++) {
        if(!is_at_start[but[i]]) {
            res += (i - at_start);
            at_start++;
            is_at_start[but[i]] = true;
        }
    }

    if(at_start == k) final_res = res;
    cout<<final_res;
    return 0;
}