#include <algorithm>
#include <iomanip>
#include <iostream>
#include <utility>
#include <vector>
using namespace std;
#define int long long
using pdd = pair<long double, long double>;
const long double INF = 1e18;
const int maxn = 505;
int n, k;
vector<pdd> vc;
long double dp[maxn][maxn];
long double sf[maxn][maxn]; // sf[i][j] 從 A[i~(n-1)] 選 j 個最小的數字
void build_sf() {
for (int i = 1; i < maxn; i++) {
fill(sf[i] + 1, sf[i] + maxn, INF);
}
for (int i = n - 1; i >= 0; i--) {
vector<long double> cur;
for (int j = i; j < n; j++) {
cur.push_back(vc[j].first);
}
sort(cur.begin(), cur.end());
sf[i][0] = 0;
for (int j = 1; j <= k; j++) {
if (j > (int)(cur.size()))
break;
else
sf[i][j] = sf[i][j - 1] + cur[j - 1];
}
}
}
long double solve(const int t) {
for (int i = 0; i < n; i++) {
fill(dp[i], dp[i] + (t + 1), INF);
}
for (int i = 0; i < n; i++) {
if (i == 0) {
dp[i][0] = vc[i].first / (t + 1.0);
dp[i][1] = vc[i].second;
continue;
}
for (int j = 0; j <= t; j++) {
if (j == 0) {
dp[i][0] = dp[i - 1][0] + vc[i].first / (t + 1.0);
} else {
long double x = dp[i - 1][j] + vc[i].first / (t + 1.0);
long double y = dp[i - 1][j - 1] + vc[i].second * 1.0 / j;
dp[i][j] = min(x, y);
}
}
}
// if (k == n) return dp[n - 1][t];
long double res = sf[0][k];
for (int i = 0; i < n; i++) {
if (k - (i+1) >= 0) {
res = min(res, dp[i][t] + sf[i + 1][k - (i+1)] / (t + 1));
}
}
return res;
}
signed main() {
ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
cout << fixed << setprecision(15);
cin >> n >> k;
for (int i = 0; i < n; i++) {
int x, y;
cin >> x >> y;
if (y == -1) {
vc.push_back({x, INF});
} else {
vc.push_back({x, y});
}
}
sort(vc.begin(), vc.end(),
[](pdd a, pdd b) { return a.second < b.second; });
build_sf();
long double res = sf[0][k];
for (int t = 1; t <= k; t++) {
res = min(res, solve(t));
}
cout << res << '\n';
return 0;
}
