#include <iostream>
#include <iomanip>
#include <vector>
#include <cmath>
#include <algorithm>
#include <set>
#include <queue>
#include <map>
#include <unordered_map>
#include <stack>
#include <bitset>
#include <string>
#include <cstring>
#include <iterator>
#include <random>
#define ll long long
#define ld long double
#define inf (ll)(2*1e18)
#define sort(a) sort(a.begin(), a.end())
#define reverse(a) reverse(a.begin(), a.end())
#define pb push_back
#define endl "\n"
using namespace std;
void solve(){
ll n, k, i, x, w, cnt, left, j, g, sum=0;
ld res=inf;
cin>>n>>k;
vector<pair<ll, ll>> a(n);
vector<vector<ld>> dp(k+1, vector<ld>(k+1, inf));
for(i=0;i<n;++i){
cin>>x>>w;
if(w == -1) w = inf;
a[i] = {w, x};
sum += x;
}
sort(a);
dp[0][0] = sum;
for(i=1;i<=k;++i){
dp[i][0] = (ld)sum/(ld)(i+1);
}
for(i=0;i<n;++i){
if(a[i].first == inf) break;
for(j=k;j>0;--j){
for(g=j;g>0;--g){
dp[j][g] = min(dp[j][g], dp[j][g-1] + (ld)a[i].first/(ld)g - (ld)a[i].second/(ld)(j+1));
}
}
}
for(i=0;i<=k;++i){
res = min(res, dp[i][i]);
}
cout<<fixed<<setprecision(20)<<res<<endl;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
srand(time(nullptr));
ll t=1;
// cin>>t;
for(;t>0;--t){
solve();
}
return 0;
}
