#include <bits/stdc++.h>
#define int long long
using namespace std;
string to_string(string s) { return s; }
template <typename T> string to_string(T v) {
bool first = true;
string res = "[";
for (const auto &x : v) {
if (!first)
res += ", ";
first = false;
res += to_string(x);
}
res += "]";
return res;
}
void dbg_out() { cout << endl; }
template <typename Head, typename... Tail> void dbg_out(Head H, Tail... T) {
cout << ' ' << to_string(H);
dbg_out(T...);
}
#ifdef DEBUG
#define dbg(...) cout << "(" << #__VA_ARGS__ << "):", dbg_out(__VA_ARGS__)
#else
#define dbg(...)
#endif
using F = long double;
const int MAXN = 501;
const F INF = 1e18;
F dp[2][MAXN];
signed main(void) {
ios_base::sync_with_stdio(false);
cin.tie(0);
int nbEtats, nbVoulus;
cin >> nbEtats >> nbVoulus;
vector<pair<int, int>> etats(nbEtats);
int cnt = 0;
for (auto &[b, a] : etats) {
cin >> a >> b;
if (b == -1)
b = INF;
else
cnt++;
}
sort(etats.begin(), etats.end());
F sol = INF;
for (int c = 0; c <= cnt and c <= nbVoulus; ++c) {
for (int i = 0; i < 2; ++i)
for (int j = 0; j < MAXN; ++j)
dp[i][j] = INF;
dp[0][0] = 0;
F s = INF;
for (int nbVus = 0; nbVus <= nbEtats; ++nbVus) {
int cur = nbVus % 2;
int nxt = cur ^ 1;
for (int i = 0; i < MAXN; ++i)
dp[nxt][i] = INF;
for (int prisA = 0; prisA <= nbVus and prisA <= nbVoulus - c; ++prisA) {
int prisB = nbVus - prisA;
if (prisB > c)
continue;
F v = dp[cur][prisA];
if (prisB == c) {
vector<int> restants;
for (int i = nbVus; i < nbEtats; ++i)
restants.push_back(etats[i].second);
sort(restants.begin(), restants.end());
for (int i = 0; i < nbVoulus - nbVus; ++i)
v += restants[i] / (F)(c + 1);
s = min(s, v);
} else if (nbVus < nbEtats) {
auto [b, a] = etats[nbVus];
// on prend en A
dp[nxt][prisA + 1] = min(dp[nxt][prisA + 1], v + a / (F)(c + 1));
// on prend en b
dp[nxt][prisA] = min(dp[nxt][prisA], v + b / (F)(prisB + 1));
}
}
}
dbg(c, s);
sol = min(sol, s);
}
cout << setprecision(9) << fixed << sol << endl;
}
