#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
const int N = 500;
const ld INF = 1e16;
int n, k;
pair<ld, ld> c[N + 10];
bool is1Subtask = 1;
ll w[(1 << 20)][20];
int main() {
ios::sync_with_stdio(0);
cin.tie(0);cout.tie(0);
cin >> n >> k;
for(int i = 1; i <= n; i++) {
cin >> c[i].first >> c[i].second;
if(c[i].second == -1) c[i].second = INF;
is1Subtask &= (c[i].second == c[i].first || c[i].second == INF);
}
sort(c + 1, c + n + 1, [](pair<ld, ld> x, pair<ld, ld> y) {return x.second < y.second;});
if(is1Subtask) {
vector<vector<ld>> dp(n + 1, vector<ld>(n + 1, INF));
dp[0][1] = 0;
for(int i = 1; i <= n; i++) {
vector<vector<ld>> newdp = dp;
for(int k = 1; k <= i; k++) {
for(int v = 1; v <= i + 1; v++) {
newdp[k][v] = min(newdp[k][v], dp[k - 1][v] + c[i].first / v);
if(v > 1 && c[i].second != INF)
newdp[k][v] = min(newdp[k][v], dp[k - 1][v - 1] + c[i].second / (v - 1)) ;
}
}
dp = newdp;
}
ld res = INF;
for(int v = 1; v <= n; v++) {
res = min(res, dp[k][v]);
}
cout << setprecision(9) << fixed << res ;
return 0;
}
if(n <= 20) {
ld res = INF;
for(int msk = 0; msk < (1 << n); msk++) {
vector<ll> v;
vector<ll> w(n + 1, -1);
ld sum = 0, veloc = 1;
for(int i = 0; i < n; i++) {
if(msk >> i & 1) {
sum += c[i + 1].second / veloc;
veloc++;
} else {
v.push_back(c[i + 1].first);
}
}
sort(v.begin(), v.end());
for(int i = 0; i <= k - veloc; i++)
sum += v[i] / veloc;
res = min(res, sum);
}
cout << setprecision(9) << fixed << res ;
return 0;
}
}
