#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 double ld;
typedef vector<ll> vll;
typedef vector<int> vi;
typedef pair<int,int> ii;
const ld INF = 1e18;
const int N = 500;
template<typename T>
void chmin(T &x, T v) {
if (x > v) x = v;
}
pair<ld, int> dp[N];
const ld EPS = 1e-4;
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) {
ld lo = -1000 / ld(c + 1) - 1, hi = 1;
ld currAns = INF;
while (hi - lo > EPS) {
ld mid = (lo + hi) / 2;
forn(a, c + 1) dp[a] = {INF, INF};
dp[0] = {0, 0};
forn(i, n) dforn(a, c + 1) {
pair<ld, int> curr = dp[a];
curr.fst += ld(x[i].fst) / (a + 1);
chmin(dp[a + 1], curr);
curr = dp[a];
curr.fst += mid + ld(x[i].snd) / (c + 1);
curr.snd--;
chmin(dp[a], curr);
}
dp[c].snd *= -1;
dp[c].fst -= mid * (k - c);
if (dp[c].snd >= k - c) {
lo = mid;
currAns = dp[c].fst;
} else {
hi = mid;
}
}
chmin(ans, currAns);
}
cout << fixed << setprecision(10) << ans << '\n';
return 0;
}
