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#include <iostream>
#include <vector>
#include <set>

using namespace std;

int main() {
    int n, k;
    cin >> n >> k;
    vector<int> a(n);
    set<int> s;
    int whereLast = -1;
    for(int i = 0; i < n; i++){
        cin >> a[i];
        s.insert(a[i]);
        if(whereLast == -1 && s.size() == k)
            whereLast = i;
    }
    if(whereLast == -1){
        cout << -1;
        return 0;
    }
    s.clear();
    int result = 0;
    for(int i = 0; i < n; i++){
        if(s.count(a[i])){
            result += whereLast - i;
            whereLast--;
        }else
            s.insert(a[i]);
    }
    cout << result;
    return 0;
}