#include <bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
#define FOR(i, l, r, d) for(int i=(l); i<=(r); i+=(d))
#define szof(x) ((int)(x).size())
#define vi vector<int>
#define pii pair<int, int>
#define F first
#define S second
#define pb push_back
#define eb emplace_back
#define mkp make_pair
const int INF = INT_MAX;
const int LNF = INF*INF;
const int MOD = 1000000007;
const int mod = 998244353;
const double eps = 1e-12;
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
const int MAX = 510;
struct State{
double A, B;
};
int n, k;
State state[MAX];
double res;
double dp[MAX][MAX];
bool cmp_by_B(State x, State y){
if(x.B == y.B) return (x.A < y.A);
return (x.B < y.B);
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin>>n>>k;
FOR(i, 1, n, 1){
cin>>state[i].A>>state[i].B;
if(state[i].B == -1) state[i].B = INF;
}
sort(state+1, state+n+1, cmp_by_B);
res = INF;
// enum. n. of collaborators
FOR(C, 0, k, 1){
int V = k - C;
// C collaborators, V votes
FOR(i, 0, C, 1){
FOR(j, 0, V, 1){
dp[i][j] = INF;
}
}
dp[0][0] = 0;
FOR(x, 1, n, 1){
for(int i = C; i >= 0; i--){
for(int j = V; j >= 0; j--){
if(i > 0) dp[i][j] = min(dp[i][j], dp[i-1][j] + state[x].B / i);
if(j > 0) dp[i][j] = min(dp[i][j], dp[i][j-1] + state[x].A / (C+1));
}
}
}
res = min(res, dp[C][V]);
}
cout<<fixed<<setprecision(12)<<res<<'\n';
return 0;
}
