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#include <bits/stdc++.h>
#define ll long long
#define mp make_pair
#define fi first
#define se second
#define pb push_back
#define vi vector<int>
#define pi pair<int, int>
#define mod 998244353
template<typename T> bool chkmin(T &a, T b){return (b < a) ? a = b, 1 : 0;}
template<typename T> bool chkmax(T &a, T b){return (b > a) ? a = b, 1 : 0;}
ll ksm(ll a, ll b) {if (b == 0) return 1; ll ns = ksm(a, b >> 1); ns = ns * ns % mod; if (b & 1) ns = ns * a % mod; return ns;}
using namespace std;
int n;
const int maxn = 500005;
int hv[maxn];
int main() {    
    int k;
    cin >> n >> k;
    vi c;
    for (int i = 1; i <= n; i++) {
        int a;
        scanf("%d", &a);
        if (hv[a]) continue;
        else {
            hv[a] = 1, c.pb(i);
        }
    }
    if (c.size() < k) cout << -1 << endl;
    else {
        ll ans = 0;
        for (int i = 0; i < k; i++)
            ans += c[i] - i - 1;
        cout << ans << endl;
    }
    return (0-0); // <3 cxr
}