#include<bits/stdc++.h>
typedef int ll;
#define pii pair<ll, ll>
#define fi first
#define se second
#define endl '\n'
#define TASK ""
#define N 503
#define LOG 17
using namespace std;
bool ghuy4g;
const ll inf = 1e9;
ll n, k;
pii a[N];
bool cmp(pii a, pii b) {
return a.se < b.se;
}
double dp[N][N][N];
void minimize(double &X, double val) {
if (X == 0) X = val;
else {
X = min(X, val);
}
}
void sub3() {
double ans = inf;
dp[1][0][0] = 0;
for (int i = 1; i <= n + 1; i ++) {
for (int j = 0; j <= i - 1; j ++) {
for (int cnt = 0; cnt <= j; cnt ++) {
if (dp[i][j][cnt] == 0 && (i != 1 && j != 0 && cnt != 0)) {
dp[i][j][cnt] = 1e9;
continue;
}
if (dp[i][j][cnt] == 1e9) continue;
if (i == n + 1) {
if (j >= k) ans = min(ans, dp[i][j][cnt]);
continue;
}
// lam thang i -> A
double nxt_x = dp[i][j][cnt] + double(a[i].fi) / (cnt + 1);
minimize(dp[i + 1][j + 1][cnt], nxt_x);
// lam den i -> B
if (a[i].se != inf) {
nxt_x = dp[i][j][cnt] + double(a[i].se) / (cnt + 1);
minimize(dp[i + 1][j + 1][cnt + 1], nxt_x);
}
// ko lam gi
minimize(dp[i + 1][j][cnt], dp[i][j][cnt]);
}
}
}
cout << fixed << setprecision(6) << ans;
}
bool klinh;
signed main() {
// freopen("test.inp", "r", stdin);
//freopen("test.out", "w", stdout);
//srand(time(0));
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n >> k;
for (int i = 1; i <= n; i ++) {
cin >> a[i].fi >> a[i].se;
if (a[i].se == -1) a[i].se = inf;
}
sort(a + 1, a + 1 + n, cmp);
sub3();
cerr << fabs(&klinh - &ghuy4g) / double(1024 * 1024);
}
