#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define debug(...) 42
#endif
const int N = 505, inf = 1e9;
int n, k;
int a[N], b[N], ord[N];
double dp[N][N];
double solve(int m) {
for (int i = 0; i <= n; ++i) {
fill(dp[i], dp[i] + k + 1, inf);
}
dp[0][0] = 0;
for (int i = 0; i < n; ++i) {
for (int j = 0; j <= min(i, k - m); ++j) {
if (dp[i][j] != inf) {
int id = ord[i + 1];
if (j + 1 <= k - m) {
dp[i + 1][j + 1] = min(dp[i + 1][j + 1], dp[i][j] + (double) a[id] / (m + 1));
}
int pick = i - j;
if (m - pick > 0) {
if (b[id] != inf) {
dp[i + 1][j] = min(dp[i + 1][j], dp[i][j] + (double) b[id] / (pick + 1));
}
} else {
dp[i + 1][j] = min(dp[i + 1][j], dp[i][j]);
}
}
}
}
return dp[n][k - m];
}
int main() {
ios::sync_with_stdio(false); cin.tie(nullptr);
cin >> n >> k;
int can = n;
for (int i = 1; i <= n; ++i) {
cin >> a[i] >> b[i];
if (b[i] == -1) {
b[i] = inf;
--can;
}
}
iota(ord + 1, ord + n + 1, 1);
sort(ord + 1, ord + n + 1, [&](int i, int j) {
return b[i] < b[j];
});
double res = solve(1);;
for (int cnt = 0; cnt <= min(can, k); ++cnt) {
res = min(res, solve(cnt));
}
cout << fixed << setprecision(15) << res;
return 0;
}
