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#include<bits/stdc++.h>
using namespace std;
#define d(a)cout<<#a<<" = "<<a<<"\n";
#define f first
#define s second
#define mp make_pair
#define TURBO ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define ll long long
#define MAXN 500500
bool a[MAXN];
int main(){
    TURBO;
    int n,k;
    cin>>n>>k;
    int ile=0;
    int sco=0;
    for(int i=0;i<n;i++){
        int x;
        cin>>x;
        if(ile<k and a[x]==0){
            sco+=(i-ile);
            ile++;
        }
        a[x]=1;
    }
    if(ile<k)cout<<-1;
    else cout<<sco;
    
}