#include <bits/stdc++.h>
using namespace std;
void fastIO(){ios_base::sync_with_stdio(false),cin.tie(0);}
#define int long long
#define ld long double
const int MAXN = 502;
int N, K;
struct State{ld vote, colab;};
bool cmpStates (const State& a, const State& b) {
if (a.colab == -1) return false;
else if (b.colab == -1) return true;
else return a.colab < b.colab;
}
vector<State> states;
struct Situ{
ld sumVote, sumColab;
bool operator> (const Situ& other) const {
if (sumVote+sumColab > other.sumVote+other.sumColab) return true;
else if (sumVote+sumColab < other.sumVote+other.sumColab) return false;
else return sumColab > other.sumColab;
}
};
Situ dp[2][MAXN][MAXN];
ld ans = (ld)1e18;
signed main() {
fastIO();
cin >> N >> K;
states.assign(N, State{(ld)0,(ld)0});
for (int i = 0; i < N; i++) {
cin >> states[i].vote >> states[i].colab;
}
sort(states.begin(), states.end(), cmpStates);
// for (int i = 0; i < N; i++) cerr << states[i].vote << " " << states[i].colab << "\n";
for (int numVote = 0; numVote <= K; numVote++) {
for (int numColab = 0; numColab <= K; numColab++) {
dp[0][numVote][numColab] = Situ{(ld)1e18, (ld)1e18};
dp[1][numVote][numColab] = Situ{(ld)1e18, (ld)1e18};
}
}
dp[0][0][0] = Situ{0, 0};
for (int idx = 0; idx < N; idx++) {
for (int numVote = 0; numVote <= K; numVote++) {
for (int numColab = 0; numColab <= K; numColab++) {
Situ situOn = dp[idx%2][numVote][numColab];
if (dp[(idx+1)%2][numVote][numColab] > Situ{situOn.sumVote, situOn.sumColab}) {
dp[(idx+1)%2][numVote][numColab] = situOn;
}
if (numVote+1 <= K && dp[(idx+1)%2][numVote+1][numColab] > Situ{situOn.sumVote + states[idx].vote, situOn.sumColab}) {
dp[(idx+1)%2][numVote+1][numColab] = situOn;
dp[(idx+1)%2][numVote+1][numColab].sumVote += states[idx].vote;
}
if (numVote+1 <= K && numColab+1 <= K && states[idx].colab != -1 && dp[(idx+1)%2][numVote+1][numColab+1] > Situ{situOn.sumVote, situOn.sumColab + states[idx].colab/(numColab+1)}) {
dp[(idx+1)%2][numVote+1][numColab+1] = situOn;
dp[(idx+1)%2][numVote+1][numColab+1].sumColab += states[idx].colab/(numColab+1);
}
}
}
}
for (int numColab = 0; numColab <= K; numColab++) {
ans = min(ans, dp[N%2][K][numColab].sumColab + dp[N%2][K][numColab].sumVote/(numColab+1));
}
cout << fixed << setprecision(12) << ans << "\n";
}
