#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll INF = 1e9+7;
const ll MOD = 998244353;
typedef pair<ll,ll> ii;
#define iii pair<ll,ii>
#define f(i,a,b) for(ll i = a;i < b;i++)
#define pb push_back
#define vll vector<ll>
#define F first
#define S second
#define all(x) (x).begin(), (x).end()
///I hope I will get uprating and don't make mistakes
///I will never stop programming
///sqrt(-1) Love C++
///Please don't hack me
///@TheofanisOrfanou Theo830
///Think different approaches (bs,dp,greedy,graphs,shortest paths,mst)
///Stay Calm
///Look for special cases
///Beware of overflow and array bounds
///Think the problem backwards
///Training
double dp[505][505][505];
ll n,k;
vector<ii>ar;
double solve(ll idx,ll i,ll j){
if(j >= k){
return 0;
}
if(idx == n){
return 1e18;
}
if(dp[idx][i][j] != -1){
return dp[idx][i][j];
}
double ans = solve(idx + 1,i,j);
ans = min(ans,solve(idx + 1,i,j+1) + (double)(-ar[idx].S) / (double)(i));
ans = min(ans,solve(idx + 1,i+1,j+1) + (double)(ar[idx].F) / (double)(i));
return dp[idx][i][j] = ans;
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cin>>n>>k;
f(i,0,n+5){
f(j,0,n+5){
f(u,0,n+5){
dp[i][j][u] = -1;
}
}
}
ll a[n],b[n];
f(i,0,n){
cin>>a[i]>>b[i];
if(b[i] == -1){
b[i] = 1e18;
}
ar.pb(ii(b[i],-a[i]));
}
sort(all(ar));
cout<<fixed<<setprecision(6)<<solve(0,1,0)<<"\n";
}
