#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define prn printf
#define scn scanf
#define pii pair<int,int>
#define fi first
#define se second
#define mp make_pair
const int nax = 500;
const int inf = 1e9;
int n,k;
double memo[nax+5][nax+5][nax+5];
vector<pii> a;
// double dp(int idx, int col, int rem) {
// if(rem == 0) return 0;
// if(idx == n) return inf;
// double &ret = memo[idx][col][rem];
// if(ret!=0) return ret;
// ret = dp(idx+1, col, rem); // ga ambil
// if(a[idx].fi != inf) ret = min(ret, (double)a[idx].fi/col + dp(idx+1, col+1, rem-1));
// else ret = min(ret, (double)a[idx].se/col + dp(idx+1,col,rem-1));
// // cerr << idx << " " << col << " " << rem << " = " << ret << endl;
// return ret;
// }
double dp (int idx, int col, int vote, int tcol) {
if(col + vote == 0) return 0;
if(idx==n) return inf;
double &ret = memo[idx][col][vote];
if(ret!=0) return ret;
ret = dp(idx+1, col, vote, tcol);
if(col>0 && a[idx].fi != inf) ret = min(ret, (double)a[idx].fi/(tcol-col+1) + dp(idx+1, col-1, vote, tcol)); //consider jd collaborator
if(vote>0) ret = min(ret, (double)a[idx].se/(tcol+1) + dp(idx+1,col,vote-1, tcol));
//cerr << idx << " " << col << " " << vote << endl;
return ret;
}
signed main () {
scn("%d%d", &n, &k);
a.assign(n,{0,0});
for(int i=0; i<n; i++) {
scn("%d%d", &a[i].se, &a[i].fi);
if(a[i].fi == -1) a[i].fi = inf;
}
sort(a.begin(), a.end());
//for(int i=0; i<n; i++) //cerr << a[i].fi << " " << a[i].se << endl;
double ans = inf;
for(int c = 0; c <= min(n,k); c++) {
//cerr << " :: " << c << endl;
for(int i=0;i<n;i++) for(int j=0;j<n; j++) for(int k=0; k<n; k++) memo[i][j][k] = 0;
ans = min(ans, dp(0, c, k-c, c));
}
prn("%f", ans);
}
/*
dp(i,j,k) = time minimum untuk visit k kota up to index i
ddn j collaboratior
dp(i,j,0) = 0
ambil b
dp(i+1, j+1, k-1) + b[i]
ambil a
dp(i+1, j, k-1) + a[i]
ga ambil
dp(i+1, j+1, k)
*/
