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#include <cstdio>
#include <algorithm>

bool used[1000005];

int main()
{
    std::fill(used, used+1000005, false);
    
    int n, k;
    scanf("%d%d", &n, &k);
    
    int x;
    long long int ans = 0;
    for (int i = 0, j = 0; i < n; i++)
    {
        scanf("%d", &x);
        if (!used[x])
        {
            used[x] = true;
            ans += i-j;
            if (++j == k)
            {
                printf("%lld\n", ans);
                return 0;
            }
        }
    }
    printf("-1\n");
    return 0;
}