#include <bits/stdc++.h>
#define int long long
#define IO ios::sync_with_stdio(0), cin.tie(0)
#define FOR(i, a, b) for (int i = a; i <= b; i++)
using namespace std;
using pii = pair<int, int>;
const int N = 505;
const int INF = 1e9;
const double dINF = 1e9;
pii a[N];
signed main() {
IO;
int n, tar;
cin >> n >> tar;
FOR(i, 1, n) {
cin >> a[i].first >> a[i].second;
if (a[i].second == -1)
a[i].second = INF;
}
sort(a + 1, a + n + 1, [&](pii p, pii q) {
return p.second < q.second;
});
vector<vector<double>> dp(n + 1, vector<double>(n + 1, dINF));
dp[0][0] = 0;
for (int i = 1; i <= n; i++) {
vector<vector<double>> ndp(n + 1, vector<double>(n + 1, dINF));
for (int j = 0; j < i; j++) {
for (int k = 0; k <= j; k++) {
if (dp[j][k] < dINF) {
ndp[j][k] = min(ndp[j][k], dp[j][k]);
if (a[i].second < INF)
ndp[j + 1][k + 1] = min(ndp[j + 1][k + 1], dp[j][k] + (double)a[i].second / (k + 1));
ndp[j + 1][k] = min(ndp[j + 1][k], dp[j][k] + (double)a[i].first / (k + 1));
}
}
}
swap(dp, ndp);
}
double ans = dINF;
for (int k = 0; k <= n; k++)
ans = min(ans, dp[tar][k]);
cout << fixed << setprecision(15) << ans << "\n";
return 0;
}
