#include <bits/stdc++.h>
using namespace std;
const int maxn = 501;
pair <int, int> a[maxn];
float dp[maxn][maxn][maxn];
int main()
{
cout << setprecision(20);
int n, k;
cin >> n >> k;
for (int i = 1;i <= n;i++)
{
cin >> a[i].second >> a[i].first;
if (a[i].first == -1)
{
a[i].first = maxn * maxn * maxn;
}
}
sort(a + 1 , a + n + 1);
for (int x = 0;x <= n;x++)
{
for (int i = 0;i <= n;i++)
{
for (int j = 0;j <= n;j++)
{
dp[x][i][j] = 1e18;
}
}
}
dp[0][0][0] = 0;
float ans = 1e18;
for (int c = 0;c <= k;c++)
{
for (int i = 1;i <= n;i++)
{
for (int x = 0;x <= min(i, c);x++)
{
for (int xp = 0;xp <= min(i - x, k - x);xp++)
{
dp[i][x][xp] = dp[i - 1][x][xp];
if (x)
{
dp[i][x][xp] = min(dp[i][x][xp], dp[i - 1][x - 1][xp] + (float)a[i].first / x);
}
if (xp)
{
dp[i][x][xp] = min(dp[i][x][xp], dp[i - 1][x][xp - 1] + (float)a[i].second / (c + 1));
}
}
}
}
ans = min(ans, dp[n][c][k - c]);
for (int i = 1;i <= n;i++)
{
for (int x = 0;x <= min(i, c);x++)
{
for (int xp = 0;xp <= min(i - x, k - x);xp++)
{
dp[i][x][xp] = 1e18;
}
}
}
}
cout << ans;
}
