#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int n, x, cnt=0;
vector<array<ll, 2> > a;
double dp[505][505][505];
double f(int c) {
for(int i=0; i<=n; i++)
for(int j=0; j<=x; j++)
for(int k=0; k<=c; k++)
dp[i][j][k] = 1e18;
dp[0][0][0] = 0;
for(int i=1; i<=n; i++) {
for(int j=0; j<=x; j++) {
int k = min(j, c);
dp[i][j][k] = dp[i-1][j][k];
if(j)
dp[i][j][k] = min(dp[i][j][k], dp[i-1][j-1][k] + (double)a[i][1] / (1 + c));
if(k && a[i][0] != 1e9)
dp[i][j][k] = min(dp[i][j][k], dp[i-1][j-1][k-1] + (double)a[i][0] / k);
}
}
return dp[n][x][c];
}
signed main() {
cin >> n >> x;
a.resize(n+1);
for(int i=1; i<=n; i++) {
cin >> a[i][1] >> a[i][0];
if(a[i][0] != -1) cnt++;
else a[i][0] = 1e9;
}
sort(a.begin()+1, a.end());
int l=0, r=min(x, cnt)-1, p = min(x, cnt);
while(l <= r) {
int mid = (l + r) / 2;
if(f(mid) <= f(mid+1)) p = mid, r = mid - 1;
else l = mid + 1;
}
cout << setprecision(3) << fixed << f(p) << '\n';
}
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