#include <bits/stdc++.h>
#define For(i,a,b) for(int i=a;i<=b;i++)
#define Forr(i,a,b) for(int i=a;i>=b;i--)
#define F first
#define S second
#define sz(x) ((int)x.size())
#define all(x) x.begin(),x.end()
#define eb emplace_back
#define INF (LL)(9e18)
#define int LL
using namespace std;
using LL=long long;
using pii=pair<int,int>;
const int MAXN=510;
double p[MAXN];
int a[MAXN];
int b[MAXN];
int mn[MAXN][MAXN];
double dp[MAXN][MAXN];
int32_t main(){
ios::sync_with_stdio(false);
cin.tie(0);
// OAO
int n,k; cin>>n>>k;
{
vector<pii> v(n);
for(auto &i:v) cin>>i.F>>i.S;
sort(all(v),[](const pii &pa,const pii &pb){
if(pa.S==-1) return false;
if(pb.S==-1) return true;
return pa.S<pb.S;
});
For(i,0,n-1){
a[i+1]=v[i].F;
b[i+1]=v[i].S;
}
}
For(i,1,n) p[i]=1.0/i;
For(i,0,n){
priority_queue<int> pq;
int sum=0;
Forr(j,n,1){
sum+=a[j];
pq.emplace(a[j]);
if(sz(pq)>i){
sum-=pq.top();
pq.pop();
}
mn[i][j]=sum;
}
}
// For(i,0,n) For(j,1,n) cout<<mn[i][j]<<" \n"[j==n];
For(i,0,n) For(j,0,n) dp[i][j]=INF;
dp[0][0]=0;
double ans=mn[k][1];
For(nc,0,k-1){
For(i,1,n) For(j,0,min(i,nc)){
dp[i][j]=dp[i-1][j]+a[i]*p[nc+1];
if(b[i]!=-1 && j){
dp[i][j]=min(dp[i][j],dp[i-1][j-1]+b[i]*p[j]);
}
if(j==nc){
ans=min(ans,dp[i][j]+mn[k-i][i+1]*p[nc+1]);
}
}
}
cout<<fixed<<setprecision(15);
cout<<ans<<"\n";
return 0;
}
