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#include <bits/stdc++.h>
 
#define INF 2147483647
#define LINF 9223372036854775807
#define NINF -2147483648
#define NLINF -9223372036854775808
#define M 1000000007
#define M1 998244353
#define A 26
#define K 31
#define P 2137
 
using namespace std;
using db=double;
using dbl=long double;
using ll=long long;
using pi=pair<int,int>;
using pl=pair<ll,ll>;
using vi=vector<int>;
using vl=vector<ll>;
using gr=vector<vector<int> >;
using grl=vector<vector<ll> >;

 
#define fp(x, a, b) for (int (x) = (a); (x) < (b); (x)++)
#define f(x, n) for (int (x) = 0; (x) < (n); (x)++)
#define fnp(x, a, b) for (int (x) = (b) - 1; (x) >= (a); (x)--)
#define fn(x, n) for (int (x) = (n - 1); (x) >= 0; (x)--)
#define sgn(x) (x) > 0 ? 1 : (x) == 0 ? 0 : -1
#define gcd(a, b) __gcd((a), (b))
#define lcm(a, b) (a) * (b) / gcd((a), (b))
#define x first
#define y second
#define mp make_pair 
#define pb push_back
#define s(x) x.size()
#define all(x) x.begin(), x.end()
#define ans(x) cout<<(x)<<"\n"
#define yes printf("YES\n")
#define no printf("NO\n")
#define fl cout.flush()
#define debarr(x, n) f(i, (n)){cout<<(x)[i]<<" ";}cout<<"\n"
#define debgr(x, n) f(i, (n)){f(j, s((x)[i])){cout<<(x)[i][j]<<" ";}cout<<"\n";}

 
mt19937 rnd(chrono::high_resolution_clock::now().time_since_epoch().count());


 
void input();
void compute();
 
int main()
{
	int T = 1;
	//scanf("%d", &T);
	while(T--)
	{
		input();
		compute();
	}
	return 0;
}

#define N 500311
 
ll n;
ll k;
ll tab[N];
ll r;
ll w;
bool vis[N];

void input()
{
	cin >> n >> k;
	f(i, n)
	{
		cin >> tab[i];
		vis[i + 1] = false;
	}
}
void compute()
{
	r = 0;
	w = 0;
	f(i, n)
	{
		if(!vis[tab[i]])
		{
			vis[tab[i]] = true;
			w += i - r;
			r++;
		}
		if(r == k)
		{
			cout<<w<<"\n";
			return;
		}
	}
	cout<<-1<<"\n";
}