#include <bits/stdc++.h>
using namespace std;
#define forn(i,n) for(int i=0;i<int(n);i++)
#define forsn(i,s,n) for(int i=int(s);i<int(n);i++)
#define dforn(i,n) for(int i=int(n)-1;i>=0;i--)
#define dforsn(i,s,n) for(int i=int(n)-1;i>=int(s);i--)
#define fst first
#define snd second
#define pb push_back
#define eb emplace_back
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
typedef long long ll;
typedef long double ld;
typedef vector<ll> vll;
typedef vector<int> vi;
typedef pair<int,int> ii;
const ld INF = 1e18;
const int N = 500;
void chmin(ld &x, ld v) {
if (x > v) x = v;
}
ld dp[N][N];
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
int n, k;
cin >> n >> k;
vector<pair<ll, ll>> x(n);
forn(i, n){
cin >> x[i].snd >> x[i].fst;
if (x[i].fst == -1) x[i].fst = 1e9;
}
sort(all(x));
ld ans = INF;
forn(c, k + 1) {
forn(a, k + 1 - c) forn(b, c + 1) dp[a][b] = INF;
dp[0][0] = 0;
forn(i, n) dforn(a, k + 1 - c) dforn(b, c + 1) {
chmin(dp[a + 1][b], dp[a][b] + ld(x[i].snd) / (c + 1));
chmin(dp[a][b + 1], dp[a][b] + ld(x[i].fst) / (b + 1));
}
chmin(ans, dp[k - c][c]);
}
cout << fixed << setprecision(10) << ans << '\n';
return 0;
}
