#include <bits/stdc++.h>
#define FOR(i,n) for(int i = 0; i<n; ++i)
#define FOREACH(v, a) for(auto & v : (a))
#define X first
#define Y second
#define PR std::pair
#define MPR std::make_pair
typedef long long ll;
typedef std::vector<int> VI;

using namespace std;

#pragma region Ready functions

int fastPower(int a, int b){
    int res=1, pop_a = a;
    while(b){
        if(b&1) res *= pop_a;
        pop_a *= pop_a;
        b>>=1;
    }
    return res;
}

#pragma endregion

int closestPos[500001];
priority_queue<pair<ll,int>> marki;

int main(){
    ios_base::sync_with_stdio(0); cin.tie(0);
    

    int n,k;

    cin>>n>>k;

    //cout<<n<<endl;

    FOR(i, n + 1) closestPos[i] = 9999999;

    for(int i = 0; i<n; i++){
        int x;
        cin>>x;
        closestPos[x] = min(closestPos[x], i);
    }

    for(int i = 1; i<=n; i++){
        if(closestPos[i] == 9999999) continue;
        marki.push({-1*closestPos[i], -1*i});
    }

    ll moves = 0;
    ll rozne = 0;

    //cout<<marki.size()<<endl;

    while(marki.size() && rozne < k){

        moves += -1*marki.top().first - rozne;
        rozne++;
        marki.pop();
    }

    if(rozne < k){
        cout<<-1;
    }
    else{
        cout<<moves;
    }



    
}