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#include <iostream>

using namespace std;

unsigned long a[500000], n;
int b[500000];

int main()
{
    unsigned long k, i = 0, j, bi, l, ret = 0;
    cin >> n >> k;
    while (i < n) cin >> a[i++];
 
    i = j = 0;
    while (k && i < n) {
        if (!b[a[i]]) {
            b[a[i]] = 1;
            ret += i - j++;
            k--;
        }
        i++;
    }
    
    if (k) { cout << -1; return 0; }

    cout << ret;
    return 0;
}