// 01001100 01001111 01010100 01000001 \\
// \\
// ╦ ╔═╗╔╦╗╔═╗ \\
// ║ ║ ║ ║ ╠═╣ \\
// ╩═╝╚═╝ ╩ ╩ ╩ \\
// \\
// 01001100 01001111 01010100 01000001 \\
#include <bits/stdc++.h>
using namespace std;
#define nl '\n'
#define ff first
#define ss second
#define ll long long
#define ld long double
#define terminator main
#define pll pair<ll,ll>
#define add insert
#define append push_back
#define pii pair<int,int>
#define all(x) (x).begin(),(x).end()
#define L0TA ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
const int N = 512;
int n, k;
bool vis[N];
double a[N];
double b[N];
vector<pair<double, int>> v, u;
double get(int x){
double ans = 0;
double p = 1;
for(int i = 0; i < x; i++){
ans += u[i].ff / p;
vis[u[i].ss] = 1;
p++;
}
int cnt = x;
for(int i = 0; cnt < k; i++){
if(vis[v[i].ss]) continue;
ans += v[i].ff / p;
cnt++;
}
for(int i = 0; i < n; i++)
vis[i] = 0;
return ans;
}
void solve(){
cin >> n >> k;
cout << fixed << setprecision(15);
for(int i = 0; i < n; i++){
cin >> a[i] >> b[i];
v.append({a[i], i});
if(b[i] >= a[i])
u.append({b[i], i});
}
sort(all(v));
sort(all(u));
double ans = 1e18;
for(int i = 0; i <= min(k, int(u.size())); i++)
ans = min(ans, get(i));
cout << ans;
}
int terminator(){
L0TA;
int T = 1;
//cin >> T;
while(T--)
solve();
return 0;
}
