#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
int n, k;
cin >> n >> k;
//~ cerr << "A\n";
vector<array<ld, 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(begin(a), end(a));
ld res = LLONG_MAX;
auto chmin = [&](ld& x, ld y) -> void {
x = min(x, y);
};
//~ cerr << "a\n";
for (int take = 0; take <= min(k, cnt); ++take) {
vector<vector<vector<ld>>> dp(2, vector<vector<ld>>(n + 1, vector<ld>(n + 1, 1e18)));
dp[0][0][0] = 0;
for (int j = 0; j < n; ++j) {
int p = j & 1;
for (int l = 0; l <= k; ++l) {
for (int z = 0; z <= min(take, l); ++z) {
dp[p ^ 1][l][z] = dp[p][l][z];
if (l > 0) {
chmin(dp[p ^ 1][l][z], dp[p][l - 1][z] + a[j][1] / (take + 1));
}
if (a[j][0] != -1 && z > 0) {
assert(l > 0);
chmin(dp[p ^ 1][l][z], dp[p][l - 1][z - 1] + a[j][0] / z);
}
}
}
}
//~ cerr << take << " " << k << "\n";
res = min(res, dp[n & 1][k][take]);
}
cout << fixed << setprecision(3) << res << "\n";
}
