#include<bits/stdc++.h>
using namespace std;
#define pii pair<int,int>
#define fi first
#define se second
const int N = 5e2+5;
int n,k;
double dp[N][N];
pii a[N];
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> n >> k;
for(int i = 1; i <= n; i++){
cin >> a[i].fi >> a[i].se;
swap(a[i].fi,a[i].se);
if(a[i].fi == -1) a[i].fi = 1e9;
}
sort(a+1,a+n+1);
double ans = 1e9;
for(int t = 0; t <= k; t++){
int kaori = k-t;
for(int i = 0; i <= n; i++){
for(int j = 0; j <= min(i,kaori); j++) dp[i][j] = 1e9;
}
dp[0][0] = 0;
for(int i = 1; i <= n; i++){
for(int j = 0; j <= min(i,kaori); j++){
if(j > 0) dp[i][j] = min(dp[i][j],dp[i-1][j-1]+(double)a[i].se/(t+1));
if(i > j && i-j <= t) dp[i][j] = min(dp[i][j],dp[i-1][j]+(double)a[i].fi/(i-j));
else if(i-j > t) dp[i][j] = min(dp[i][j],dp[i-1][j]);
}
}
ans = min(ans,dp[n][kaori]);
}
cout << fixed << setprecision(18) << ans;
}
