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#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,n) for (int i=a;i<n;i++)
#define per(i,a,n) for (int i=n-1;i>=a;i--)
#define pb push_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
typedef vector<int> VI;
typedef long long ll;
typedef pair<int,int> PII;
typedef double db;
const ll mod=1000000007;
ll powmod(ll a,ll b) {ll res=1;a%=mod; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%mod;a=a*a%mod;}return res;}
ll gcd(ll a,ll b) { return b?gcd(b,a%b):a;}
// head


int n,k,x;
ll ans,bad;
set<int> hs;
int main() {
	scanf("%d%d",&n,&k);
	rep(i,0,n) {
		scanf("%d",&x);
		if (SZ(hs)<k&&!hs.count(x)) {
			ans+=bad;
			hs.insert(x);
		} else {
			bad++;
		}
	}
	if (SZ(hs)!=k) {
		puts("-1");
	} else {
		printf("%lld\n",ans);
	}
}