#include <bits/stdc++.h>
using namespace std;
#define fi first
#define sc second
const int MAXN = 505;
const double INF = 1e6;
pair<double, double> v[MAXN]; // b a
double dp[2][MAXN][MAXN];
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
int n,k; cin>>n>>k;
for(int i = 1; i <= n; i++){
cin>>v[i].sc>>v[i].fi;
if(v[i].fi == -1)v[i].fi = INF;
}
sort(v+1,v+1+n);
for(int i = 1; i <= k; i++){
dp[0][n+1][i] = INF;
dp[1][k+1][i] = INF;
}
double ans = INF;
for(int ini = 0; ini <= k; ini++){
for(int i = n; i >= 1; i--)
for(int y = 1; y <= k-ini; y++)
dp[0][i][y] = min(dp[0][i+1][y], dp[0][i+1][y-1] + v[i].sc/double(ini+1));
for(int i = k; i >= 1; i--){
dp[1][i][0] = dp[0][i][k-i+1];
for(int x = 1; x <= ini; x++){
dp[1][i][x] = dp[1][i+1][x] + v[i].sc/double(ini+1);
if(v[i].fi != -1)dp[1][i][x] = min(dp[1][i][x], dp[1][i+1][x-1] + v[i].fi/double(ini-x+1));
}
}
ans = min(ans, dp[1][1][ini]);
}
cout<<fixed<<setprecision(15)<<ans<<"\n";
}
