#include <bits/stdc++.h>
#include <experimental/random>
#include <random>
//#include <ext/pb_ds/assoc_container.hpp>
//using namespace __gnu_pbds;
using namespace std;
using ld = double;
using ll = long long;
const int INF = 1e9, MOD = 1e9 + 7;
void solve();
signed main() {
#ifdef LOCAL
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
ios::sync_with_stdio(false);
cin.tie(0);
int q = 1;
//cin >> q;
while (q--) {
solve();
}
}
vector<pair<int, int>> a;
int n, k;
ld get(int x) {
vector<vector<ld>> dp(x + 1, vector<ld>(k - x + 1, INF));
dp[0][0] = 0;
int col = 1;
for (auto [sec, fir] : a) {
vector<vector<ld>> ndp(x + 1, vector<ld>(k - x + 1, INF));
for (int i = 0; i <= min(x, col); i++) {
for (int j = 0; j <= min(col - i, k - x); j++) {
ndp[i][j] = dp[i][j];
if (i != 0 && sec != -1) {
ndp[i][j] = min(ndp[i][j], dp[i - 1][j] + (ld) sec / (ld) i);
}
if (j != 0) {
ndp[i][j] = min(ndp[i][j], dp[i][j - 1] + (ld) fir / (ld) (x + 1));
}
}
}
dp = ndp;
col++;
}
return dp[x][k - x];
}
void solve() {
cin >> n >> k;
int cnt = 0;
for (int i = 0; i < n; i++) {
int x, y; cin >> x >> y;
if (y != -1) cnt++;
a.emplace_back(y, x);
}
sort(a.begin(), a.end());
ld ans = INF;
int l = 0, r = min(k, cnt);
while (r - l > 2) {
int m1 = l + (r - l) / 3;
int m2 = r - (r - l) / 3;
ld f1 = get(m1), f2 = get(m2);
ans = min({ans, f1, f2});
if (f1 < f2) {
r = m2;
} else {
l = m1;
}
}
for (int i = l; i <= r; i++) {
ans = min(ans, get(i));
}
cout << fixed << setprecision(9) << ans;
}
