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#include <bits/stdc++.h>
using namespace std;
#define REP(i,a,b) for (int i = (a); i <= (b); ++i)
#define REPD(i,a,b) for (int i = (a); i >= (b); --i)
#define FORI(i,n) REP(i,1,n)
#define FOR(i,n) REP(i,0,int(n)-1)
#define mp make_pair
#define pb push_back
#define pii pair<int,int>
#define vi vector<int>
#define ll long long
#define SZ(x) int((x).size())
#define DBG(v) cerr << #v << " = " << (v) << endl;
#define FOREACH(i,t) for (typeof(t.begin()) i=t.begin(); i!=t.end(); i++)
#define fi first
#define se second

const int N = 500500;

int n,k,m,l;
int a[N], vis[N], badids[N], goodids[N];

int main() {
	scanf("%d%d", &n, &k);
	FOR(i,n) scanf("%d", &a[i]);
	FOR(i,n) {
		if (!vis[a[i]]) {
			if (i >= k) {
				goodids[l++] = i;
			}
			vis[a[i]] = 1;
		} else if (i < k) {
			badids[m++] = i;
		}
	}
	if (l < m) {
		printf("-1\n");
		return 0;
	}
	//cerr << m << " do zmiany\n";
	//FOR(i,m) cerr << a[badids[i]] << " "; cerr << "\n";
	//FOR(i,m) cerr << a[goodids[i]] << " "; cerr << "\n";
	ll res = 1LL * m * m;
	FOR(i,m) {
		res += k - m + i - badids[i];
		res += goodids[i] - (k + i);
		//cerr << badids[i] << " : " << k - m + i - badids[i] << "\n";
		//cerr << goodids[i] << " : " << goodids[i] - (k + i) << "\n";
	}
	printf("%lld\n", res);
	return 0;
}