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#define make_pair mp
#define emplace_back pb
#include <bits/stdc++.h>
using namespace std;
mt19937 mt_rand(time(0));
const int N = 5e5 + 5;
int n, k, tab[N];
bool is[N];
int main() {
	scanf("%d%d", &n, &k);
    for(int i=1;i<=n;i++) {
        scanf("%d", &tab[i]);
        is[tab[i]] = true;
    }
    int cnt = 0;
    for(int i=1;i<=n;i++) cnt += is[i];
    if(cnt < k) {
        printf("-1\n");
        return 0;
    }
    for(int i=1;i<=n;i++) is[i] = 0;
    cnt = 0;
    long long res = 0;
    for(int i=1;i<=k;i++) {
        if(is[tab[i]]) {
            cnt++;
            res += k - i;
        }
        is[tab[i]] = 1;
    }
    long long cnt2 = cnt;
    long long cc = (cnt2 - 1) * cnt2 / 2;
    res -= cc;
    cnt = k - cnt + 1;
    if(cnt <= k)
    for(int i=k+1;i<=n;i++) {
        if(!is[tab[i]]) {
            is[tab[i]] = 1;
            res += i - cnt;
            cnt++;
            if(cnt == k + 1) break;
        }
    }
    printf("%lld\n", res);
	return 0;
}