/**
____ ____ ____ ____ ____ ____
||l |||e |||i |||n |||a |||d ||
||__|||__|||__|||__|||__|||__||
|/__\|/__\|/__\|/__\|/__\|/__\|
**/
#include<bits/stdc++.h>
#define endl '\n'
using namespace std;
typedef long long ll;
void speed()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
}
const int maxn = 510, smalln = 110;
struct city
{
int a, b, idx;
}c[maxn], ct[maxn];
int n, k, used[maxn];
bool cmpa(city c1, city c2)
{
return c1.a < c2.a;
}
bool cmpb(city c1, city c2)
{
if (c1.b != -1 && c2.b != -1)
return c1.b < c2.b;
if (c1.b != -1)
return true;
if (c2.b != -1)
return false;
return c1.idx < c2.idx;
}
double dp[maxn][maxn], fp[maxn][maxn];
/// dp[i][j][f] - smallest time up to the i-th city using j votes and f collaborators
void solve()
{
cin >> n >> k;
int col = 0;
for (int i = 1; i <= n; i ++)
{
cin >> c[i].a >> c[i].b;
if (c[i].b != -1)
col ++;
c[i].idx = i;
ct[i] = c[i];
}
sort(c + 1, c + n + 1, cmpb);
double ans = 1e18;
for (int goal = 0; goal <= min(k, col); goal ++)
{
for (int i = 0; i <= n; i ++)
for (int f = 0; f <= goal; f ++)
fp[i][f] = 1e18;
fp[0][0] = 0.0;
for (int i = 1; i <= n; i ++)
for (int f = 0; f <= goal; f ++)
{
fp[i][f] = min(fp[i][f], fp[i - 1][f] + c[i].a / (double)(goal + 1.0));
if (f >= 1 && c[i].b != -1)
{
fp[i][f] = min(fp[i][f], fp[i - 1][f - 1] + c[i].b / (double)(f));
}
}
for (int i = 0; i <= n + 1; i ++)
for (int j = 0; j <= k; j ++)
dp[i][j] = 1e18;
dp[n + 1][0] = 0.0;
for (int i = n; i > 0; i --)
for (int j = 0; j <= k; j ++)
{
dp[i][j] = dp[i + 1][j];
if (j > 0)
dp[i][j] = min(dp[i][j], dp[i + 1][j - 1] + c[i].a / (double)(goal + 1.0));
}
for (int i = 0; i <= k; i ++)
{
double sum = fp[i][goal] + dp[i + 1][k - i];
if (sum < ans)
ans = sum;
}
///ans = min(ans, fp[n][goal]);
}
printf("%0.6f\n", ans);
}
int main()
{
solve();
return 0;
}
