#include<bits/stdc++.h>
using namespace std;
using ll = long long;
#define all(x) (x).begin(), (x).end()
const int N = 507;
int n, k;
array<int, 2> a[N];
double dp[N][N];
int main() {
cin.tie(0)->sync_with_stdio(false);
cin >> n >> k;
for (int i = 1; i <= n; i++) {
cin >> a[i][0] >> a[i][1];
if (a[i][1] == -1)
a[i][1] = 1e9;
}
sort(a + 1, a + n + 1, [&](array<int, 2> &x, array<int, 2> &y) { return x[1] < y[1]; });
double ans = 1e9;
for (int c = 0; c <= k; c++) {
for (int i = 0; i <= n; i++)
for (int j = 0; j <= n; j++)
dp[i][j] = 1e9;
dp[0][0] = 0;
for (int i = 1; i <= n; i++) {
for (int j = 0; j < i; j++) {
if (j < k - c)
dp[i][j + 1] = min(dp[i][j + 1], dp[i - 1][j] + (double) a[i][0] / (c + 1));
if (i - j - 1 < c && a[i][1] < 1e9)
dp[i][j] = min(dp[i][j], dp[i - 1][j] + (double) a[i][1] / (i - j));
if (i - j - 1 >= c)
dp[i][j] = min(dp[i][j], dp[i - 1][j]);
}
}
ans = min(ans, dp[n][k - c]);
}
cout << fixed << setprecision(10) << ans << '\n';
return 0;
}
