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#include <bits/stdc++.h>

using namespace std;

int main()
{
    int n, k;
    scanf("%d%d", &n, &k);
    vector<int> c(n, 0);
    uint64_t total = 0;
    for (int i = 0, j = 0; i < n; ++i) {
        int a;
        scanf("%d", &a);
        if (c[--a]++ == 0)
        {
            total += i - j;
            if (++j == k)
            {
                printf("%ld\n", total);
                return 0;
            }
        }
    }
    printf("-1\n");
    return 0;
}