#include <iostream>
#include <vector>
#include <deque>
using namespace std;
typedef long long ll;
const int maxn = 1e6 + 5;
const ll INF = 1e18;
ll v[maxn];

ll B(int i) {
    return v[i] + i;
}
int main(){
    ios_base::sync_with_stdio(false);
    int n;
    ll k;
    cin >> n >> k;

    vector<ll> pref(n + 1, 0);
    for(int i = 0; i < n; i++) {
        cin >> v[i];
    }

    for(int i = 0; i < n; i++) {
        pref[i + 1] = pref[i] + v[i];
    }

    deque<int> dq;
    ll res = INF;

    for(int i = 0; i < n; i++) {
        while(!dq.empty() and B(dq.back()) >= B(i)) {
            dq.pop_back();
        }
        dq.push_back(i);
        int p = i - k + 1;
        if(p < 0) {
            continue;
        }

        while(!dq.empty() and dq.front() < p) {
            dq.pop_front();
        }

        ll x_max = B(dq.front()) - p;
        if(x_max < k) {
            continue;
        }

        ll sum = pref[p + k] - pref[p];
        ll to_cut = sum - (k * x_max) + (k * (k - 1) / 2);
        ll t = 10LL * p + 5LL * to_cut;

        res = min(res, t);

    }

    if(res == INF){
        res = -1;
    }
    cout << res << endl;
    return 0;
}