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#include <bits/stdc++.h>

using namespace std;

constexpr int LIM = 5e5;
bitset<LIM + 3> was;
int t[LIM + 3];

int main() {
    int n, k;
    scanf("%d%d", &n, &k);

    for(int i = 0; i < n; ++i) {
        scanf("%d", &t[i]);
    }

    long long left = 0, res = 0;
    for(int i = 0; i < n; ++i) {
        if(was[t[i]]) {
            ++left;
        } else {
            was[t[i]] = 1;
            k -= 1;
            res += left;
            if(k == 0)
                break;
        }
    }

    if(k == 0)
        printf("%lld", res);
    else
        printf("-1");
}