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#include <bits/stdc++.h>

using namespace std;

const int N = 500'007;

int n, k;
int in[N];
bool was[N];

int main() 
{
    scanf("%d %d", &n, &k);
    for (int i = 1; i <= n; ++i) {
        scanf("%d", &in[i]);
    }
    
    int picked = 0;
    long long ans = 0;
    for (int i = 1; i <= n; ++i) {
        int v = in[i];
        if (!was[v]) {
            picked++;
            was[v] = true;
        } else {
            ans += k - picked;
        }

        if (picked == k) {
            break;
        }
    }

    if (picked < k) {
        ans = -1;
    }

    printf("%lld\n", ans);
    return 0;
}