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#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define mid (l+r)/2
#define PI acos((ld)-1)
#define BOOST ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0)
typedef long long ll;
typedef long double ld;
typedef pair<int, int> ii;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<ii> vii;
typedef vector<vi> vvi;

const int N = 5e5+5;
int in[N];
bool vis[N];

int main(){
	BOOST;
	int n, k, cnt = 0;
	ll ans = 0;
	cin >> n >> k;
	for(int i=1; i<=n; i++){
		cin >> in[i];
		if(i <= k && vis[in[i]])
			ans += k-i, cnt++;
		if(cnt > 0 && i > k && !vis[in[i]])
			ans += i-k, cnt--;
		vis[in[i]] = 1;
	}
	if(cnt > 0)
		cout << -1;
	else
		cout << ans;
}