#include <bits/stdc++.h>
#define IO_OP std::ios::sync_with_stdio(0); std::cin.tie(0);
#define F first
#define S second
#define V vector
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define SZ(v) int((v).size())
#define ALL(v) (v).begin(), (v).end()
using namespace std;
typedef long long ll;
typedef pair<int, int> pi;
typedef V<int> vi;
string _reset = "\u001b[0m", _yellow = "\u001b[33m", _bold = "\u001b[1m";
void DBG() { cerr << "]" << _reset << endl; }
template<class H, class...T> void DBG(H h, T ...t) {
cerr << to_string(h);
if(sizeof ...(t)) cerr << ", ";
DBG(t...);
}
#ifdef CHEISSMART
#define debug(...) cerr << _yellow << _bold << "Line(" << __LINE__ << ") -> [" << #__VA_ARGS__ << "]: [", DBG(__VA_ARGS__)
#else
#define debug(...)
#endif
const int INF = 1e9 + 7;
vi pre[505];
signed main()
{
IO_OP;
int n, k;
cin >> n >> k;
V<pi> a(n);
for(int i = 0; i < n; i++) {
cin >> a[i].S >> a[i].F;
if(a[i].F == -1) a[i].F = INF;
}
sort(ALL(a));
for(int he = 0; he < n; he++) {
vi aux;
for(int i = he; i < n; i++)
aux.PB(a[i].S);
sort(ALL(aux));
pre[he] = aux;
}
auto solve = [&] (int be) {
// b: 1, 1/2, 1/3..., 1/be
// a: 1/(be + 1)
V<double> dp(be + 1, INF);
dp[0] = 0;
auto go = [&] (int he) {
double ans = dp[be];
vi aux = pre[he];
for(int i = 0; i < k - he; i++)
ans += 1. * aux[i] / (be + 1);
return ans;
};
double ans = INF;
ans = min(ans, go(0));
for(int i = 0; i < k; i++) {
V<double> ndp(be + 1, INF);
for(int j = 0; j <= be; j++) if(dp[j] != INF) {
// a
ndp[j] = min(ndp[j], dp[j] + 1. * a[i].S / (be + 1));
// b
if(j + 1 <= be)
ndp[j + 1] = min(ndp[j + 1], dp[j] + 1. * a[i].F / (j + 1));
}
swap(dp, ndp);
ans = min(ans, go(i + 1));
}
return ans;
};
double ans = INF;
for(int be = 0; be <= k; be++)
ans = min(ans, solve(be));
cout << fixed << setprecision(10) << ans << '\n';
}
