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#define pb push_back
#define mp make_pair
#define fi first
#define se second 
#define all(...) begin(__VA_ARGS__) , end(__VA_ARGS__)
#define boost {ios_base::sync_with_stdio(false); cin.tie(); cout.tie();}

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef vector <int> vi;
typedef pair<int,int> PII;
typedef pair<ll,ll> PLL;
constexpr ll nax = 5e5+6969, INF = 2e9+6969;

int n, k, cnt = 0;
bitset <nax> vis;
ll ans = 0;

int main()
{
	scanf("%d%d", &n, &k);
	for(int i=0;i<n;i++) 
	{
		int x;
		scanf("%d", &x);
		if(vis[x] || cnt == k) continue;
		vis[x] = 1;
		cnt++;
		ans += i + 1 - cnt;
	}
	if(cnt < k) puts("-1");
	else printf("%lld\n", ans);
	
	return 0;
}