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#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

 
#pragma GCC optimize("Ofast")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#define mp make_pair
#define pb push_back
#define ppb pop_back
#define pf push_front
#define ppf pop_front
#define eb emplace_back
#define st first
#define nd second
#define vt vector
#define VAR(__var)  #__var << ": " << __var << " "
#define PAIR(__var) #__var << ": " << __var.first << ", " << __var.second << " " 
#define FOR(__var, __start, __end)  for(int __var=__start; __var<__end; ++__var)
#define FORB(__var, __start, __end) for(int __var=__start; __var>__end; --__var)
#define maxi(__x, __y) __x = (__x>__y?__x:__y)
#define mini(__x, __y) __x = (__x<__y?__x:__y)
#define all(__var)     (__var).begin(),(__var).end()
#define rall(__var)    (__var).rbegin(),(__var).rend()
#define sz(__var)      (int)(__var).size()
#define satori         int __test; cin>>__test; while(__test--)
// flaga -DDEBUG
#ifndef DEBUG
#define DEBUG 0
#endif
#define debug if(DEBUG)

using namespace std;

using namespace __gnu_pbds;
template <typename T>
using ord_set = tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>;

typedef     int16_t i16;
typedef     int32_t i32;
typedef     int64_t i64;
typedef        void  u0;
typedef        bool  u1;
typedef        char  u8;
typedef    uint16_t u16;
typedef    uint32_t u32;
typedef    uint64_t u64;
typedef       float f32;
typedef      double f64;
typedef long double f86;

const i32 INF = 1e9+2137;
const i64 mod = 1420696969;
// 1696969 1e9+69 1237
//templatke (patrz góra^) wzialem od wiktora przed oiem:)
//gambit zadanka
int32_t main()
{
	ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
	int n,k;
	cin>>n>>k;
	vector<int>v(n);
	int ans=0;
	int y=0;
	while(n--)
	{
		int x;
		cin>>x;
		v[x]++;
		if(v[x]==1 and k>0)
		{
			k--;
			ans+=y;
		}else y++;
	}
	if(k)cout<<-1;
	else cout<<ans;
}