#include <bits/stdc++.h>
using namespace std;
#define chinatsu ios_base::sync_with_stdio(false);
#define kano cin.tie(nullptr); cout.tie(nullptr)
#define int long long
#define double long double
#define fi first
#define se second
#define pb push_back
double dp[2][501][501];
signed main(){
chinatsu kano;
int n, k;
cin >> n >> k;
vector<array<double, 2>> a(n);
int cnt = 0;
for (int i = 0; i < n; ++i) {
for (int j = 0; j < 2; ++j) {
cin >> a[i][j];
}
cnt += (a[i][1] != -1);
swap(a[i][0], a[i][1]);
}
sort(a.begin(), a.end());
int left = 0, right = min(k, cnt) - 1;
int res = min(k, cnt);
auto calc = [&](int take){
for (int i = 0; i < 2; ++i) {
for (int j = 0; j <= k - take; ++j) {
for (int z = 0; z <= take; ++z) {
dp[i][j][z] = 1e18;
}
}
}
dp[0][0][0] = 0;
for (int j = 0; j < n; ++j) {
int p = j & 1;
for (int l = 0; l <= k - take; ++l) {
for (int z = 0; z <= take; ++z) {
dp[p ^ 1][l][z] = dp[p][l][z];
if (l > 0) {
dp[p ^ 1][l][z] = min(dp[p ^ 1][l][z], dp[p][l - 1][z] + a[j][1] / (take + 1));
}
if (a[j][0] != -1 && z > 0) {
dp[p ^ 1][l][z] = min(dp[p ^ 1][l][z], dp[p][l][z - 1] + a[j][0] / z);
}
}
}
}
return dp[n & 1][k - take][take];
};
while (left <= right) {
int m = (left + right) / 2;
if (calc(m) <= calc(m + 1)) {
right = m - 1;
res = m;
} else left = m + 1;
}
cout << fixed << setprecision(15) << calc(res) << endl;
return 0;
}
