#include <bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
#define FOR(i, l, r, d) for(int i=(l); i<=(r); i+=(d))
#define szof(x) ((int)(x).size())
#define vi vector<int>
#define pii pair<int, int>
#define F first
#define S second
#define pb push_back
#define eb emplace_back
#define mkp make_pair
const int INF = INT_MAX;
const int LNF = INF*INF;
const int MOD = 1000000007;
const int mod = 998244353;
const double eps = 1e-12;
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
const int MAX = 510;
struct State{
double A, B;
};
int n, k;
State state[MAX];
double res;
bool cmp_by_A(State x, State y){
return (x.A < y.A);
}
bool cmp_by_B(State x, State y){
return (x.B < y.B);
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin>>n>>k;
FOR(i, 1, n, 1){
cin>>state[i].A>>state[i].B;
if(state[i].B == -1) state[i].B = INF;
}
res = INF;
// enum. n. of collaborators
FOR(i, 0, k, 1){
// i collaborators
sort(state+1, state+n+1, cmp_by_B);
sort(state+i+1, state+n+1, cmp_by_A);
double sum = 0;
double col = 1;
FOR(j, 1, i, 1){
sum += state[j].B / col;
col += 1;
}
FOR(j, i+1, k, 1){
sum += state[j].A / col;
}
res = min(res, sum);
//cerr<<"i = "<<i<<" : sum = "<<sum<<endl;
}
cout<<fixed<<setprecision(12)<<res<<'\n';
return 0;
}
