#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<double, double> pll;
#define fastio ios::sync_with_stdio(false), cin.tie(0)
#pragma GCC optimize("Ofast")
#define pb push_back
#define eb emplace_back
#define f first
#define s second
#define lowbit(x) x&-x
const int maxn = 2e5 + 5;
const double INF = 1e18;
//enumerate number of b from 0 ... k - 1
//it is optimal to get collaboraters first then give speeches to k - #b people
//for a set of collaboraters, it is optimal to sort them by increasing bs
double dp[505][505];
int main(void){
fastio;
int n, k;
cin>>n>>k;
vector<pll> a(n);
for(int i = 0; i < n; i++){
cin>>a[i].f>>a[i].s;
if(a[i].s == -1) a[i].s = INF;
}
sort(a.begin(), a.end(), [&](pll a, pll b){ return a.s < b.s || (a.s == b.s && a.f < b.f);});
double best = INF;
for(int goal = 0; goal < k; goal++){ // enumerate number of collaboraters(can also tenary search)
for(int i = 0; i <= n; i++) for(int j = 0; j <= goal; j++) dp[i][j] = INF;
dp[0][0] = 0;
for(int i = 0; i < n; i++){
for(int j = 0; j < goal; j++){
dp[i + 1][j] = min(dp[i + 1][j], dp[i][j] + a[i].f / (goal + 1));
dp[i + 1][j + 1] = min(dp[i + 1][j + 1], dp[i][j] + a[i].s / (j + 1));
}
}
for(int i = 0; i <= n; i++){
//choose k - i elements for second set
vector<double> cand;
for(int j = i; j < n; j++) cand.pb(a[j].f);
sort(cand.begin(), cand.end());
double cur = dp[i][goal];
for(int j = 0; j < k - i; j++) cur += cand[j] / (goal + 1);
best = min(best, cur);
}
}
cout<<fixed<<setprecision(5)<<best<<"\n";
}
