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#include <bits/stdc++.h>
 
using namespace std;

int main(){
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    long long n, k, ans = 0, y = 0;
    cin >> n >> k;
    vector<int> tab(n + 7);
    for(int i = 1; i <= n; ++i){
        cin >> tab[i];
    }
    int hm = 1;
    if(hm == k){
        cout << ans;
        return 0;
    }
    int last = tab[2], lastx = 2;
    map<int, int> mp;
    mp[tab[2]]++;
    if(tab[1] != tab[2]){
        y = 2;
        ++hm;
    }
    for(int i = 3; i <= k; ++i){
        if(tab[i] != last && tab[i] != tab[1] && mp[tab[i]] == 0){
            ans += i - lastx;
            lastx++;
            last = tab[i]; 
            mp[tab[i]]++;
            ++hm;
        }
        if(mp[tab[i]] == 1){
            ++y;
        }
    }
    if(y == k){
        cout << 0;
        return 0;
    }
    if(hm == k){
        cout << ans;
        return 0;
    }
    for(int i = k + 1; i <= n; ++i){
        if(hm == k){
            cout << ans;
            return 0;
        }
        if(tab[i] != last && tab[i] != tab[1] &&  mp[tab[i]] == 0){
            ans += i - lastx;
            lastx++;
            last = tab[i];
            mp[tab[i]]++;
            ++hm;
        }
    }
    if(hm == k){
        cout << ans;
        return 0;
    }
    cout << -1;
}