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#pragma GCC optimize ("Ofast")
#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
#define FOR(i, a, b) for (auto i=(a); i<(b); i++)
#define FORD(i, a, b) for (int i=(a); i>(b); i--)
#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(), (x).end()
#define PPC(x) __builtin_popcountll(x)
#define MSB(x) (63 - __builtin_clzll(x))
#define LSB(x) __builtin_ctz(x)
#define ARG(x, i) (get<i>(x))
#define ithBit(m, i) ((m) >> (i) & 1)
#define pb push_back
#define ft first
#define sd second
#define kw(a) ((a) * (a))
#ifdef DEBUG
#include "debug.h"
#else
#define dbg(...) 0
#endif
using namespace std; 

template <typename T1, typename T2> inline void remin(T1& a, T2 b) { a = min(a, (T1)b);	}
template <typename T1, typename T2> inline void remax(T1& a, T2 b) { a = max(a, (T1)b);	}
 
const int maxN = 1 << 19;

bool vis[maxN];

void solve()
{
	int n, k, juz = 0;
	scanf ("%d%d", &n, &k);
	long long res = 0;
		
	FOR(i, 0, n)
	{
		int x;
		scanf ("%d", &x);
		if (vis[x])
			continue;
		vis[x] = true;
		res += i - juz++;
		
		if (juz == k)
			break;
	}
	
	printf("%lld\n", juz == k ? res : -1ll);
}
 
int main()
{
	int t = 1;
	//scanf ("%d", &t);
	FOR(tid, 1, t+1)
	{
		//printf("Case #%d: ", tid);
		solve();
	}
	return 0;
}