#include <bits/stdc++.h>
using namespace std;
#define int long long
const int MAX = 5e2 + 5;
int n, k;
pair<int, int> state[MAX];
double f[2][MAX][MAX];
namespace SUB12 {
bool realko() {
return n > 7;
}
void solve() {
sort(state + 1, state + n + 1, [](const pair<int, int> &a, const pair<int, int> &b) -> bool {
if (a.second != -1 && b.second != -1) {
return 2 * a.second - a.first < 2 * b.second - b.first;
}
return a.second > b.second;
});
for (int i = 0; i < 2; i++) {
for (int j = 0; j <= n + 1; j++) {
for (int z = 0; z <= k; z++) {
f[i & 1][j][z] = 1e9;
}
}
}
f[0][1][0] = 0.0;
for (int i = 1; i <= n; i++) {
for (int j = 1; j <= i + 1; j++) {
for (int z = 0; z <= k; z++) {
f[i & 1][j][z] = 1e9;
}
}
for (int j = 1; j <= i + 1; j++) {
for (int z = 0; z <= k; z++) {
f[i & 1][j][z] = f[(i - 1) & 1][j][z];
if (z) f[i & 1][j][z] = min(f[i & 1][j][z], f[(i - 1) & 1][j][z - 1] + (1.0 * state[i].first / j));
if (j > 1 && z && state[i].second != -1) {
f[i & 1][j][z] = min(f[i & 1][j][z], f[(i - 1) & 1][j - 1][z - 1] + (1.0 * state[i].second / (j - 1)));
}
}
}
}
double ans = 1e9;
for (int j = 1; j <= n + 1; j++) {
ans = min(ans, f[n & 1][j][k]);
}
cout << fixed << ans;
exit(0);
}
}
int32_t main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
// freopen("test.inp", "r", stdin); freopen("test.out", "w", stdout);
cin >> n >> k;
for (int i = 1; i <= n; i++) cin >> state[i].first >> state[i].second;
if (SUB12::realko()) SUB12::solve();
vector<int> p(n);
iota(p.begin(), p.end(), 1);
double ans = 1e9;
do {
for (int i = 0; i < 2; i++) {
for (int j = 0; j <= n + 1; j++) {
for (int z = 0; z <= k; z++) {
f[i & 1][j][z] = 1e9;
}
}
}
f[0][1][0] = 0.0;
for (int i = 1; i <= n; i++) {
int cur = p[i - 1];
for (int j = 1; j <= i + 1; j++) {
for (int z = 0; z <= k; z++) {
f[i & 1][j][z] = 1e9;
}
}
for (int j = 1; j <= i + 1; j++) {
for (int z = 0; z <= k; z++) {
f[i & 1][j][z] = f[(i - 1) & 1][j][z];
if (z) f[i & 1][j][z] = min(f[i & 1][j][z], f[(i - 1) & 1][j][z - 1] + (1.0 * state[cur].first / j));
if (j > 1 && z && state[cur].second != -1) {
f[i & 1][j][z] = min(f[i & 1][j][z], f[(i - 1) & 1][j - 1][z - 1] + (1.0 * state[cur].second / (j - 1)));
}
}
}
}
double res = 1e9;
for (int j = 1; j <= n + 1; j++) {
res = min(res, f[n & 1][j][k]);
}
if (res - ans < 0) {
ans = res;
// cout << fixed << ans << ' ';
// for (int i : p) cout << i << ' ';
// cout << '\n';
}
} while (next_permutation(p.begin(), p.end()));
cout << fixed << ans;
return 0;
}
