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#include <bits/stdc++.h>
using namespace std;

#ifdef D
#define DEBUG(x)        \
    do {                \
        x               \
        cout.flush();   \
    } while (0)
#else
#define DEBUG(x)
#endif

const int MAXN = 5e5+7;

int n, k, a;
long long ans;
vector<int> r;
bool vis[MAXN];

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> n >> k;
    for (int i = 0; i < n; ++i) {
        cin >> a;
        if (!vis[a]) {
            vis[a] = true;
            r.push_back(i);
        }
    }
    if (r.size() < k) {
        ans = -1;
    } else {
        for (int i = 0; i < k; i++) {
            ans += r[i] - i;
        }
    }
    cout << ans;
    return 0;
}