#include <bits/stdc++.h>
using namespace std;
using ll = long long;
//#define int ll
using P = pair<int, int>;
#define all(x) x.begin(), x.end()
#define rep(x,s,e) for (auto x=(s)-((s)>(e));x!=(e)-((s)>(e));((s)<(e)?x++:x--))
#define sz(x) (int)x.size()
const char nl = '\n';
const int mod = 998244353;
bool cmp(P a, P b) {
if (a.second != -1 && b.second != -1)return a.second < b.second;
else if (a.second != -1)return 1;
else if (b.second != -1)return 0;
return a.first < b.first;
}
void mnu(double &x, double y) {
if (y == -1)return;
if (x ==-1 || y < x)x = y;
}
void solve() {
int n, k; cin >> n >> k;
vector<P> a(n);
for (auto &i: a)cin >> i.first >> i.second;
sort(all(a), cmp);
//for (auto i: a)cout << i.first << ' ' << i.second << nl;
double dp[n][n+1][n+1];
rep(i, 0, n)
rep(j, 0, n+1)
rep(p, 0, n+1)dp[i][j][p] = -1;
rep(i, 0, n) {
dp[i][1][0] = a[i].first;
if (a[i].second != -1)dp[i][1][1] = a[i].second;
dp[i][0][0] = 0;
}
rep(i, 0, n) {
rep(j, 1, i+2)
rep(cnt, 0, i+2) {
if (i)mnu(dp[i][j][cnt], dp[i-1][j][cnt]);
//if (i == n-1 && j == 3)cout << dp[i-1][j-1][cnt]+a[i].first <<nl;
for (int x = i-1; x >= 0; --x) { // bring i-th
//if (i == n-1 && j == 3) {
//cout << a[i].first<< nl;
//}
if (dp[x][j-1][cnt]== -1)continue;
mnu(dp[i][j][cnt], dp[x][j-1][cnt]+(double)a[i].first/(double)(cnt+1));
if (cnt && a[i].second > 0) // bring i-th and it'll be col;
mnu(dp[i][j][cnt], dp[x][j-1][cnt-1]+(double)a[i].second/(double)(cnt));
}
}
}
double res = -1;
//cout << dp[0][k-1][0] << nl;
rep(cnt, 0, k+1)mnu(res, dp[n-1][k][cnt]);
cout << res << nl;
//cout << setprecision(10) << fixed << res << nl;
//for (auto i: a)cout << i.first << " " << i.second << nl;
}
int32_t main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
solve();
return 0;
}
