#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>;
template<typename T>
void chmin(T &_a,const T &_b){
if(_a>_b) _a=_b;
}
const int MAXN=510;
double p[MAXN];
pii a[MAXN];
double dp[MAXN][MAXN][MAXN];
int32_t main(){
ios::sync_with_stdio(false);
cin.tie(0);
// OAO
int n,K; cin>>n>>K;
bool oao=true;
For(i,1,n){
cin>>a[i].F>>a[i].S;
if(a[i].S!=-1) oao=false;
}
if(oao){
sort(a+1,a+n+1,[](pii &pa,pii &pb){
return pa.F<pb.F;
});
int tot=0;
For(i,1,K) tot+=a[i].F;
cout<<tot<<"\n";
return 0;
}
sort(a+1,a+n+1,[](pii &pa,pii &pb){
return pa.S<pb.S;
});
// For(i,1,n) cerr<<a[i].F<<" "<<a[i].S<<"\n";
p[0]=1;
For(i,1,n) p[i]=1.0/(i+1);
For(i,0,n) For(j,0,n) For(k,0,n) dp[i][j][k]=INF;
dp[0][0][0]=0;
if(n==K){
double ans=INF;
For(nc,0,n){
For(i,1,n) For(j,0,min(i,nc)){
dp[0][i][j]=dp[0][i-1][j]+a[i].F*p[nc];
if(a[i].S!=-1 && j){
chmin(dp[0][i][j],dp[0][i-1][j-1]+a[i].S*p[j-1]);
}
}
chmin(ans,dp[0][n][nc]);
}
cout<<fixed<<setprecision(15);
cout<<ans<<"\n";
return 0;
}
double ans=INF;
For(nc,0,K){
For(i,1,n) For(j,0,min(i,K)) For(k,0,min(j,nc)){
dp[i][j][k]=dp[i-1][j][k];
if(j){
chmin(dp[i][j][k],dp[i-1][j-1][k]+a[i].F*p[nc]);
}
if(a[i].S!=-1 && k){
chmin(dp[i][j][k],dp[i-1][j-1][k-1]+a[i].S*p[k-1]);
}
}
// if(nc==3){
// For(i,1,n) For(j,0,K) For(k,0,nc){
// cerr<<dp[i][j][k]<<"\t\n"[k==nc];
// if(j==K && k==nc) cerr<<"\n";
// }
// }
// cerr<<nc<<" "<<dp[n][K][nc]<<"\n";
chmin(ans,dp[n][K][nc]);
}
cout<<fixed<<setprecision(15);
cout<<ans<<"\n";
return 0;
}
