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#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define ull  unsigned long long
#define s second
#define f first
#define mp make_pair
const int maxN = 5e5+7;
int last[maxN];
int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int x,n,k,ans=0;
    vector<int>orng;
    cin>>n>>k;
    for (int i = 1; i <= n; i++)
    {
        cin>>x;
        if(last[x]==0){last[x]=i;orng.push_back(i);}
    }
    if(orng.size()<k)
    {
        cout<<"-1";
        return 0;
    }
    for (int i = 1; i <= k; i++)
    {
        ans+=orng[i-1]-i;
    }
    cout<<ans;

}