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#include<bits/stdc++.h>
#define VAR(i,n) __typeof(n) i = (n)
#define loop(i,j,s) for(int i=j;i<s;i++)
#define loopback(i,j,s) for(int i=j;i>=s;i--)
#define foreach(i,c) for(VAR(i,(c).begin());i!=(c).end();i++)
#define pln( x ) cout << x << "\n"
#define ps( x ) cout << x << " "
#define entr cout << "\n" 
#define pcnt(i) __builtin_popcount(i)
#define ll long long
#define pb push_back
#define mp make_pair
#define ff first
#define ss second
#define SIZE(c) (c).size()
#define ALL(c) (c).begin(), (c).end()
using namespace std;
typedef vector<ll> vi;
const int INFTY=20000000;
const int MAX=500100;
const int MOD=10000000;

void coutTab(int* tab,int n){
	loop(i,0,n){
		cout<<tab[i]<<" ";
	}
	cout<<"\n";
}
//------------------------------------------
int main(){
	ios_base::sync_with_stdio(0);
    ll n,k;
    cin>>n>>k;
    vi a(n);
    loop(i,0,n) cin>>a[i];
    set<int> cat;
    ll A=0,B=0;

    // first pass
    int ca = 0;
    loop(i,0,k) {
        if(cat.find(a[i]) != cat.end()) {
            A += i;
            ca++;
        } else {
            cat.insert(a[i]);
        }
    }

    //second pass
    int cb = 0; 
    loop(i,k,n) {
        if(cb < ca && cat.find(a[i]) == cat.end()) {
            cb++;
            B += i;
            cat.insert(a[i]);
        }
    }

    if(cb < ca) {
        pln(-1);
    } else {
        pln(B-A);
    }
}