#include <bits/stdc++.h>

// #define int int64_t

using f64 = double;
using pii = std::pair<int, int>;

const int kMaxN = 505;
const f64 kEps = 1e-6;

int n, k;
pii a[kMaxN];
f64 f[kMaxN][kMaxN];

f64 func(int cnt) {
  static pii b[kMaxN];
  for (int i = 1; i <= n; ++i) b[i] = a[i];
  for (int i = 0; i <= n; ++i)
    for (int j = 0; j <= n; ++j)
      f[i][j] = 1e9;
  f[0][0] = 0;
  for (int i = 1; i <= n; ++i) {
    for (int j = 0; j <= cnt; ++j) {
      f[i][j] = f[i - 1][j] + (f64)a[i].first / (cnt + 1);
      if (j && a[i].second != -1) f[i][j] = std::min(f[i][j], f[i - 1][j - 1] + (f64)a[i].second / j);
    }
  }
  f64 ret = 1e9;
  for (int i = k + 1; i <= cnt; ++i) ret = std::min(ret, f[i][cnt]);
  for (int i = k; i >= cnt; --i) {
    std::sort(b + 1 + i, b + 1 + n);
    f64 sum = f[i][cnt];
    for (int j = i + 1; j <= k; ++j) sum += (f64)b[j].first / (cnt + 1);
    ret = std::min(ret, sum);
  }
  return ret;
}

void dickdreamer() {
  std::cin >> n >> k;
  int can = 0;
  for (int i = 1; i <= n; ++i) {
    std::cin >> a[i].first >> a[i].second;
    if (a[i].second != -1) ++can;
  }
  std::sort(a + 1, a + 1 + n, [&] (pii p1, pii p2) {
    if (p1.second != -1 && p2.second != -1) return p1.second < p2.second;
    else return p1.second > p2.second;
  });
  f64 ans = 1e9;
  for (int cnt = 0; cnt <= std::min(can, k); ++cnt) ans = std::min(ans, func(cnt));
  std::cout << std::fixed << std::setprecision(10) << ans << '\n';
}

int32_t main() {
#ifdef ORZXKR
  freopen("in.txt", "r", stdin);
  freopen("out.txt", "w", stdout);
#endif
  std::ios::sync_with_stdio(0), std::cin.tie(0), std::cout.tie(0);
  int T = 1;
  // std::cin >> T;
  while (T--) dickdreamer();
  // std::cerr << 1.0 * clock() / CLOCKS_PER_SEC << "s\n";
  return 0;
}