#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][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 = (1LL<<60);
}
sort(a+1, a+n+1);
double ans = (1LL<<60);
rep(cc,0,k) {
if (a[cc].fi == (1LL<<60)) break;
rep(i,0,n) {
rep(j,0,n) {
if (i == j) {
rep(k,0,cc) dp[i][j][k] = (1LL<<60);
} else {
dp[i][j][cc] = (1LL<<60);
}
}
}
dp[0][0][0] = 0;
rep(i,1,n) {
rep(j,0, min(i,k)) {
if (i == j) {
rep(k,0,min(j,cc)) {
dp[i][j][k] = min(dp[i][j][k], dp[i-1][j][k]);
if (j > 0 && k > 0) dp[i][j][k] = min(dp[i][j][k], dp[i-1][j-1][k-1] + (double)a[i].fi/(k));
if (j > 0) dp[i][j][k] = min(dp[i][j][k], dp[i-1][j-1][k] + (double)a[i].se/(cc+1));
}
} else {
dp[i][j][cc] = min(dp[i][j][cc], dp[i-1][j][cc]);
if (j>0) dp[i][j][cc] = min(dp[i][j][cc], dp[i-1][j-1][cc] + (double)a[i].se/(cc+1));
}
}
}
ans = min(ans, dp[n][k][cc]);
}
cout << fixed << setprecision(15) << ans << '\n';
}
