#include<bits/stdc++.h>
using namespace std;
#define IOS cin.tie(nullptr)->sync_with_stdio(0),cin.exceptions(cin.failbit);
#define lb(x) (x)&-(x)
#define all(x) (x).begin(),(x).end()
#define ll long long
#define db long double
constexpr int maxN=5e2+5;
int n,K;
db dp[maxN][maxN];
pair<db,db> val[maxN];
inline db DP(int G){
for(int i = 0;i<=K;i++)for(int j = 0;j<=G;j++)dp[i][j] = 1e18;
dp[0][0] = 0;
for(int i = 0;i<n;i++){
auto [b,a] = val[i];
for(int j = min(K,i+1);j;j--){
if(i+1==j){
for(int k = G;k>=0;k--){
dp[j][k] = min(dp[j][k],dp[j-1][k] + a/(G+1));
if(k)dp[j][k] = min(dp[j][k],dp[j-1][k-1] + b/k);
}
}
else {
dp[j][G] = min(dp[j][G],dp[j-1][G] + a/(G+1));
}
}
}
return dp[K][G];
}
int main(){
IOS
cin>>n>>K;
for(int i = 0;i<n;i++){
cin>>val[i].second>>val[i].first;
if(val[i].first==-1)val[i].first = 1e9;
}
sort(val,val+n);
db ans = 1e18;
for(int i = 0;i<K;i++)ans = min(ans,DP(i));
cout<<fixed<<setprecision(15)<<ans<<'\n';
}
