#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;
sort(v+1,v+1+n);
double ans = INF;
for(int ini = 0; ini <= k; ini++){
for(int x = 0; x <= ini; x++)
for(int y = 0; y <= k-x; y++)
if(x != 0 || y != 0)dp[(n+1)%2][x][y] = INF;
for(int i = n; i >= 1; i--)
for(int x = 0; x <= ini; x++)
for(int y = 0; y <= k-x; y++){
dp[i%2][x][y] = dp[(i+1)%2][x][y];
if(y > 0)
dp[i%2][x][y] = min( dp[i%2][x][y], dp[(i+1)%2][x][y-1] + (v[i].sc/(double(ini)+1)) );
if(x > 0 && v[i].fi != -1)
dp[i%2][x][y] = min( dp[i%2][x][y], dp[(i+1)%2][x-1][y] + (v[i].fi/(ini-x+1)) );
}
ans = min(ans, dp[1][ini][k-ini]);
}
cout<<fixed<<setprecision(15)<<ans<<"\n";
}
