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///MEMENTO MEMO, MEMENTO LONG LONG
#include <bits/stdc++.h>

#define DEBUG if(1)
#define COUT cout << "\e[36m"
#define ENDL "\e[39m" << endl
#define VAR(v) " [\e[32m" << #v << "\e[36m=\e[91m" << v << "\e[36m] "

using namespace std;

typedef long long LL;

int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int n, k;
    cin >> n >> k;
    vector<int> cnt(n+1);
    vector<int> rep(n, 1);

    for (int i = 0; i < n; ++i)
    {
        int a;
        cin >> a;
        if(cnt[a] == 0)
        {
            rep[i] = 0;
        }
        cnt[a]++;
    }
    int rep_cnt[2] = {0,0};
    LL invs = 0;
    for (int i = 0; i < n; ++i)
    {
        if(rep[i] == 0)
        {
            invs += (LL)rep_cnt[1];
        }
        rep_cnt[rep[i]]++;
        if(rep_cnt[0] >= k)
        {
            break;
        }
    }
    cout << ((rep_cnt[0] >= k)? invs : -1) << "\n";
}