#include <bits/stdc++.h>
#define pb push_back
#define fst first
#define snd second
#define fore(i,a,b) for(int i=a,pao=b;i<pao;++i)
#define SZ(x) ((int)x.size())
#define ALL(x) x.begin(),x.end()
#define mset(a,v) memset((a),(v),sizeof(a))
#define FIN ios::sync_with_stdio(0);cin.tie(0);cout.tie(0)
using namespace std;
typedef long long ll;
const int MAXN=505,INF=1e9;
int A[MAXN],B[MAXN],v[MAXN];
void chmin(double &a,double b){
if(a>b)a=b;
}
int main(){FIN;
int n,k;cin>>n>>k;
iota(v,v+n,0);
fore(i,0,n){
cin>>A[i]>>B[i];
if(B[i]==-1)B[i]=INF;
}
sort(v,v+n,[&](int a,int b){
return B[a]<B[b];
});
double ans=INF;
fore(C,0,n){
vector<vector<double>>ndp(n+1,vector<double>(C+1,INF));
ndp[0][0]=0;
fore(i,0,n){
fore(l,0,C+1){
//~ chmin(dp[i+1][j][l],dp[i][j][l]); -- no lo uso
int ii=v[i];
chmin(ndp[i+1][l],ndp[i][l]+(double)A[ii]/((double)C+1));
if(l<C)chmin(ndp[i+1][l+1],ndp[i][l]+(double)B[ii]/((double)l+1));
}
}
//~ cout<<"====================="<<endl;
//~ fore(i,0,n){
//~ fore(l,0,C+1)cout<<ndp[i][l]<<" ";
//~ cout<<endl;
//~ }
vector<vector<double>>dp(n+1,vector<double>(k+1,INF));
fore(i,0,n){
if(i<k+1)chmin(dp[i][i],ndp[i][C]);
fore(j,0,k+1){
chmin(dp[i+1][j],dp[i][j]);
int ii=v[i];
if(j<k)chmin(dp[i+1][j+1],dp[i][j]+(double)A[ii]/((double)C+1));
}
}
//~ fore(i,0,n){
//~ fore(l,0,k+1)cout<<dp[i][l]<<" ";
//~ cout<<endl;
//~ }
chmin(ans,dp[n][k]);
}
cout<<fixed<<setprecision(20)<<ans<<endl;
}
