#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MX = 505;
int N, K;
int a[MX], b[MX];
double dp[MX][MX][MX];
vector<pair<int, int>> v;
int main() {
cin.tie(0); ios_base::sync_with_stdio(0);
for(int i = 0; i < MX; i++)
for(int j = 0; j < MX; j++)
for(int k = 0; k < MX; k++)
dp[i][j][k] = 1e9;
cin >> N >> K;
for(int i = 1; i <= N; i++) {
cin >> a[i] >> b[i];
v.push_back({b[i], a[i]});
}
sort(v.begin(), v.end(), [&](auto x, auto y) {
if(x.first == -1 && y.first == -1) return x.second < y.second;
if(x.first == -1) return false;
if(y.first == -1) return true;
if(x.first - x.second == y.first - y.second) return x.first < y.first;
return x.first - x.second < y.first - y.second;
});
dp[0][0][0] = 0;
for(int i = 0; i <= N; i++) {
for(int j = 0; j <= N; j++) { // votes
for(int k = 0; k <= N; k++) { // num of collaborator
if(dp[i][j][k] == 1e9) continue;
// skip
dp[i + 1][j][k] = min(dp[i + 1][j][k], dp[i][j][k]);
// ambil vote
dp[i + 1][j + 1][k] = min(dp[i + 1][j + 1][k], dp[i][j][k] + (double) v[i].second / (k + 1));
// ambil collaborator
if(v[i].first != -1)
dp[i + 1][j + 1][k + 1] = min(dp[i + 1][j + 1][k + 1], dp[i][j][k] + (double) v[i].first / (k + 1));
}
}
}
double ans = 1e9;
for(int k = 0; k <= N; k++) {
ans = min(ans, dp[N][K][k]);
}
cout << fixed << setprecision(15) << ans << '\n';
}
