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#include <bits/stdc++.h>

using namespace std;

bool found[600001];

int main() {
    int n, k;
    cin >> n >> k;

    int fcnt = 0;
    int64_t w = 0;

    for(int i = 0; i < n && fcnt < k; i++) {
         int a;
         scanf("%d", &a);

         if(!found[a]) {
             found[a] = true;

             w += (i - fcnt);
             fcnt++;
         }
    }

    if(fcnt != k) cout << "-1\n";
    else cout << w << endl;

    return 0;
}