#include <bits/stdc++.h>
using namespace std;
const long double pi = 3.14159265358979323846;
long long MOD = 1e9 + 7;
const char nl = '\n';
const long long inf = 1e15;
long long nxt() {
  long long x;
  cin >> x;
  return x;
}
long long mul(long long x, long long y) {
  return (1ll * (x % MOD) * (y % MOD));
}
long long modpow(long long x, long long y) {
  long long z = 1;
  while (y > 0) {
    if (y % 2) z = mul(z, x);
    x = mul(x, x);
    y /= 2;
  }
  return z;
}
long long power(long long x, long long y) {
  long long z = 1;
  while (y > 0) {
    if (y % 2) z = z * x;
    x = x * x;
    y /= 2;
  }
  return z;
}
long long gcd(long long a, long long b) {
  if (a < b) return gcd(b, a);
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long min(long long a, long long b) {
  if (a < b) return a;
  return b;
}
long long max(long long a, long long b) {
  if (a > b) return a;
  return b;
}
long long sq(long long a) {
  long long ans = (1ll * a * a);
  return ans;
}
void solve() {
  long long n, k;
  cin >> n >> k;
  long long t[n];
  vector<long long> neg;
  for (long long i = 0; i < n; i++) {
    cin >> t[i];
    if (t[i] < 0) neg.push_back(i);
  }
  if ((long long)(neg.size()) > k) {
    cout << -1 << nl;
    return;
  }
  if ((long long)(neg.size()) == 0) {
    cout << 0 << nl;
    return;
  }
  if (n == 1) {
    if (t[0] < 0) {
      cout << 1 << nl;
    } else
      cout << 0 << nl;
    return;
  }
  vector<long long> dis;
  for (long long i = 0; i < (long long)(neg.size()) - 1; i++) {
    if (neg[i + 1] - neg[i] > 1) {
      dis.push_back(neg[i + 1] - neg[i] - 1);
    }
  }
  sort(((dis).begin()), ((dis).end()));
  long long ans = (long long)(dis.size()) * 2;
  ans++;
  if (t[n - 1] >= 0) ans++;
  long long ewd = k - (long long)(neg.size());
  for (long long x : dis) {
    if (ewd >= x) ewd -= x, ans -= 2;
  }
  long long ex = n - 1 - neg.back();
  if (ewd >= ex && ex) ans--;
  cout << ans << '\n';
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0), cout.tie(0);
  long long TC = 1;
  while (TC--) {
    solve();
  }
  cerr << "Time : " << 1000 * ((double)clock()) / (double)CLOCKS_PER_SEC
       << "ms\n";
}
