#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];
int n, tar;
/// 共有 cnt 個助選人
double calc(int cnt) {
vector<vector<double>> dp(n + 1, vector<double>(n + 1, dINF));
dp[0][0] = 0;
for (int i = 1; i <= n; i++) {
for (int j = tar; j >= 1; j--)
dp[j][cnt] = min(dp[j][cnt], dp[j - 1][cnt] + (double)a[i].first / (cnt + 1));
for (int j = cnt; j >= 0; j--) {
if (j != cnt)
dp[i][j] = min(dp[i][j], dp[i - 1][j] + (double)a[i].first / (cnt + 1));
if (j && a[i].second < INF)
dp[i][j] = min(dp[i][j], dp[i - 1][j - 1] + (double)a[i].second / j);
}
}
return dp[tar][cnt];
}
signed main() {
IO;
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;
});
double ans = dINF;
for (int i = 0; i <= tar; i++)
ans = min(ans, calc(i));
cout << fixed << setprecision(15) << ans << "\n";
return 0;
}
