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#include <bits/stdc++.h>

using namespace std;

#define endl '\n'
#define st first
#define nd second
#define pb push_back
#define sz(x) (int)(x).size()
#define all(x) (x).begin(), (x).end()
#define ll long long
ll mod=1000000007;
int inf=1000000007;
ll infl=1000000000000000007;

int a[500007];
bool was[500007];

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int n,k;
    cin>>n>>k;
    for(int i=1;i<=n;i++) cin>>a[i];
    int ile=0;
    ll ans=0;
    for(int i=1;i<=n;i++)
    {
        if(was[a[i]]||ile>=k) continue;
        was[a[i]]=1;
        ile++;
        ans+=(ll)(i-ile);
    }
    if(ile<k) cout<<-1;
    else cout<<ans;


    return 0;
}