#include <bits/stdc++.h>
using namespace std;
typedef long double ld;
const int N = 505;
int n, k, a[N], b[N], par[N][N];
vector<pair<int, int>> vec;
ld dp[N][N];
int main(){
cin >> n >> k;
int sm = 0;
for (int i = 0; i < n; i ++){
cin >> a[i] >> b[i];
if (b[i] == -1)
b[i] = 1e6;
sm += a[i];
vec.push_back({b[i], -a[i]});
}
sort(vec.begin(), vec.end());
for (int i = 0; i < n; i ++)
a[i] = -vec[i].second, b[i] = vec[i].first;
for (int i = 0; i <= n; i ++){
for (int j = 0; j <= n; j ++){
if (j == 0)
dp[i][j] = sm;
else if (i == 0)
dp[i][j] = 1e6;
else{
dp[i][j] = 1e6;
if (dp[i - 1][j] < dp[i][j])
par[i][j] = j, dp[i][j] = dp[i - 1][j];
if (dp[i - 1][j - 1] + (ld)b[i - 1] / (ld)j - a[i - 1] < dp[i][j]);
par[i][j] = j - 1, dp[i][j] = dp[i - 1][j - 1] + (ld)b[i - 1] / (ld)j - a[i - 1];
}
}
}
ld ans = 1e6;
for (int i = 0; i <= n; i ++){
vector<int> inds;
int ci = n, cj = i;
while (cj > 0){
if (cj != par[ci][cj])
inds.push_back(ci - 1);
cj = par[ci][cj];
ci--;
}
reverse(inds.begin(), inds.end());
ld tme = 0; int cur = 1, csm = sm;
for (int j : inds){
tme += (ld)b[j] / (ld)cur;
cur++;
csm -= a[j];
}
tme += (ld)csm / (ld)cur;
ans = min(ans, tme);
}
cout.precision(20);
cout << ans << endl;
}
