#include <bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i,a,b) for (int i = a; i <= b; ++i)
#define pii pair<int,int>
#define fi first
#define se second
const int MAXN = 510;
array<int,3> a[MAXN], b[MAXN];
bool mk[MAXN];
int32_t main() {
ios_base::sync_with_stdio(0); cin.tie(nullptr);
int n,k; cin >> n >> k;
rep(i,1,n) {
cin >> a[i][1] >> a[i][0];
a[i][2] = b[i][2] = i;
b[i][0] = a[i][1];
b[i][1] = a[i][0];
if (a[i][0]==-1) a[i][0] = 1e9;
}
sort(a+1, a+n+1);
sort(b+1, b+n+1);
double ans = 1e9;
rep(j,0,k) {
if (a[j][0] == 1e9) break;
double cur = 0;
int cnt = 1;
rep(i,1,j) {
cur += (double)a[i][0]/cnt;
mk[a[i][2]] = 1;
cnt++;
}
int cc = 0;
rep(i,1,n) {
if (cc == k-j) break;
if (mk[b[i][2]])continue;
cur += (double)b[i][0]/cnt;
cc++;
}
//cout << cur << '\n';
ans = min(ans, cur);
}
cout << fixed << setprecision(20) << ans << '\n';
}
