#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define fst first
#define snd second
#define pb push_back
#define forn(i,a,b) for(int i = a; i < b; i++)
#define ALL(x) x.begin(),x.end()
#define SZ(x) (int)x.size()
#define mset(a,v) memset((a),(v),sizeof(a))
#define FIN ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define dbg(v) cout<<"Line("<<__LINE__<<"): "<<#v<<" = "<<v<<'\n';
#define pi pair<int,int>
#define pll pair<ll,ll>
typedef int ll;
using namespace std;
using namespace __gnu_pbds;
typedef tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update> indexed_set;
typedef tree<int,null_type,less_equal<int>,rb_tree_tag,tree_order_statistics_node_update> indexed_multiset;
const int MAXN = 501;
double dp[MAXN][MAXN][MAXN];
bool vis[MAXN][MAXN][MAXN];
ll n,k,c;
vector<pair<double,double>> s;
double f(ll x, ll y, ll z){
double &res = dp[x][y][z];
if(vis[x][y][z]) return res;
vis[x][y][z]=true;
if(y==k&&z==c){ return res=(double)0;}
if(x==n) return res=(double)10000000000;
if(n-x<k-y) return res=(double)10000000000;
if(n-x<c-z) return res=(double)10000000000;
res=(double)10000000000;
res=min(res,f(x+1,y+1,z)+(s[x].snd/(c+1)));
if(z<c){
res=min(res,f(x+1,y+1,z+1)+(s[x].fst/(z+1)));
}
if(z==c) res=min(res,f(x+1,y,z));
//cout<<x<<" "<<y<<" "<<z<<" ->>>> "<<res<<'\n';
return res;
}
int main(){FIN;
cin>>n>>k;
s.clear();
s.resize(n);
forn(i,0,n){ cin>>s[i].snd>>s[i].fst; if(s[i].fst==-1){ s[i].fst=(double)10000000000000000; }}
sort(ALL(s));
//forn(i,0,n) cout<<s[i].fst<<" "<<s[i].snd<<'\n';
double rres = (double)100000000;
forn(cc,0,k+1){
forn(i,0,MAXN){
for(int j = min((ll)i+1,k); j>=0; j--){
for(int h = min(cc,j); h>=0; h--){
vis[i][j][h]=false;
}
}
}
c=cc;
rres=min(rres,f(0,0,0));
//cout<<cc<<" -------------------- "<<f(0,0,0)<<'\n';
}
//cout<<"llego\n";
cout<<fixed<<setprecision(15)<<rres<<'\n';
return 0;
}
