#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
using ll = long long;
using dd = double;
using vi = vector<int>;
const int mx = 500;
const dd INF = 1'000'000'0.0;
dd a[1+mx], b[1+mx];
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int N, K;
cin >> N >> K;
cout << std::fixed;
cout.precision(10);
b[0] = 0;
for(int i = 1; i <= N; i++) cin >> a[i] >> b[i];
vi I(1+N);
for(int i = 0; i <= N; i++) I[i] = i;
sort(I.begin(), I.end(), [] (int u, int v)
{
dd bu = b[u], bv = b[v];
if(bu == -1) bu = INF;
if(bv == -1) bv = INF;
return bu < bv;
});
// cerr << '\n';
dd A[1+N], B[1+N];
for(int i = 1; i <= N; i++) A[i] = a[I[i]], B[i] = b[I[i]];
double res = INF;
int cc = 0;
for(int i = 1; i <= N; i++) cc += (B[i] != -1);
// cerr <<"cc = " << cc << "\n";
// cerr << "\n\n\n";
// for(int i = 1; i <= N; i++) cerr << A[i] << ' ' << B[i] << '\n';
for(int x = 0; x <= min(K,cc); x++)
{
// cerr <<"\n\n\n\n x = " << x << '\n';
dd DP[1+N][1+K-x]; //last position, non-collaborators
for(int i = 0; i <= N; i++)
for(int j = 0; j <= K-x; j++)
DP[i][j] = INF;
DP[0][0] = 0.0;
for(int i = 1; i <= N; i++)
{
for(int j = 0; j <= min(i, K-x); j++)
{
// cerr << "\n\n state = " << i << ' ' << j << "/ " << i << ' ' << K-x << "\n";
if(i-j > x)
DP[i][j] = DP[i-1][j];
if(i - j <= x && B[i] != -1 && i-j > 0)
{
// cerr << "#\n";
// cerr << i-1 << ' ' << j << ' ' << i-j-1+1 << '\n';
// cerr << "A\n";
DP[i][j] = min(DP[i][j], DP[i-1][j] + (B[i] / double(i-j-1 + 1)));
}
if(j >= 1)
{
DP[i][j] = min(DP[i][j], DP[i-1][j-1] + (A[i] / double(x+1)));
}
// cerr << x << ' ' << i << ' ' << j << " : " << DP[i][j] << '\n';
}
}
res = min(res, DP[N][K - x]);
// cerr << "\n\n\n " << N << ' ' << K-x << ' ' << 1+N << ' ' << 1+x << "\n\n\n";
// cerr << "res = " << res << "\n";
}
cout << res << '\n';
}
