#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];
double dp[MAXN][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].se >> a[i].fi;
if (a[i].fi == -1) a[i].fi = 1e9;
}
sort(a+1, a+n+1);
double ans = 1e9;
rep(cc,0,k) {
if (a[cc].fi == 1e9) break;
rep(j,1,n) dp[0][j] = 1e9;
rep(i,1,cc) {
dp[i][0] = dp[i-1][0] + (double)a[i].fi/i;
rep(j,1,n) dp[i][j] = dp[i-1][j];
}
rep(i,cc+1,n) {
rep(j,0,n) {
dp[i][j] = dp[i-1][j];
if (j>0) dp[i][j] = min(dp[i][j], dp[i-1][j-1] + (double)a[i].se/(cc+1));
//cout << i << ' ' << j << ' ' << dp[i][j] << '\n';
}
}
ans = min(ans, dp[n][k-cc]);
}
cout << fixed << setprecision(15) << ans << '\n';
}
