#include <bits/stdc++.h>
using namespace std;
#define EmiliaMyWife ios::sync_with_stdio(0); cin.tie(0);
using ll = int64_t;
using ull = uint64_t;
using uint = uint32_t;
using ld = long double;
const int INF = 0x3f3f3f3f;
const int MOD = 1e9 + 7;
const ll LINF = ll(2e18) + ll(1e15);
const double EPS = 1e-8;
static int LamyIsCute = []() {
EmiliaMyWife
return 48763;
}();
const int N = 507;
double dp[N][N];
signed main() {
int n, k;
cin >> n >> k;
vector<pair<int, int>> arr(n);
int can = 0;
for(int i = 0; i < n; i++) {
cin >> arr[i].first >> arr[i].second;
if(arr[i].second == -1)
arr[i].second = INF;
else
can++;
}
sort(arr.begin(), arr.end(), [&](auto a, auto b) { return a.second < b.second; });
double ans = 1e18;
for(int i = 0; i <= min(can, k); i++) {
for(int j = 0; j < N; j++)
for(int l = 0; l < N; l++)
dp[j][l] = 1e18;
dp[0][0] = 0;
for(int j = 0; j < n; j++) {
for(int l = 0; l <= min(j, k - i); l++) {
if(dp[j][l] > 1e9)
continue;
int r = j - l;
if(r < i)
dp[j + 1][l] = min(dp[j + 1][l], dp[j][l] + double(arr[j].second) / (r + 1));
else
dp[j + 1][l] = min(dp[j + 1][l], dp[j][l]);
dp[j + 1][l + 1] = min(dp[j + 1][l + 1], dp[j][l] + double(arr[j].first) / (i + 1));
}
}
ans = min(ans, dp[n][k - i]);
}
cout << fixed << setprecision(15) << ans << '\n';
}
