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#include <bits/stdc++.h>

using namespace std;

#define rep(i, n) for(int i = 0; i < n; i++)
#define pb push_back
#define debug(x) cout << #x << " " << x << endl
#define all(x) x.begin(),x.end()
#define ll long long
#define vi vector<int>
#define vvi vector<vi>
#define vvvi vector<vvi>
#define vll vector<ll>
#define vvll vector<vll>
#define ld long double
#define ull unsigned long long
#define vull vector<ull>
#define fi first
#define se second
#define PII pair<int,int>
#define vPII vector<PII>
#define getunique(v) {sort(v.begin(), v.end()); v.erase(unique(v.begin(), v.end()), v.end());}
#define int128 __int128

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
//    cout << setprecision(16) << fixed;
//    freopen("input.txt", "r", stdin);
//    freopen("output.txt", "w", stdout);
    int tt = 1;
//    cin >> tt;
    while(tt--) {
        int n,k; cin >> n >> k;
        vi a(n), b(n+2,0);
        rep(i,n) cin >> a[i];
        int ile = 0;
        ll ans = 0;
        rep(i,n){
            if(!b[a[i]]){
                b[a[i]] = 1;
                ans += (ll)(i - ile);
                ile++;
                if(ile == k) break;
            }
        }
        if(ile != k){
            cout << -1;

        }else{
            cout << ans;
        }
    }
}