#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
#define pb push_back
#define all(v) (v).begin(), (v).end()
const int MAXN = 510;
pii a[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].fi >> a[i].se;
double ans = 1e9;
rep(mask,1,(1<<n)-1) {
double cur = 0;
vector<int> b,c;
rep(i,1,n) {
if (mask&(1<<(i-1))) b.pb(a[i].se);
else c.pb(a[i].fi);
}
if (b.size() > k) continue;
sort(all(b));
sort(all(c));
int cnt = 1;
for (int &v : b) {
cur += (double)v/cnt;
cnt++;
}
rep(i,0,k-(int)b.size()-1) {
cur+=(double)c[i]/cnt;
}
ans = min(ans, cur);
}
cout << fixed << setprecision(15) << ans << '\n';
}
