#include <bits/stdc++.h>
#define int long long
#define double long double
#define MAX 510
#define MOD 1000000007
#define INF 0x7f7f7f7f7f7f7f7f
#define endl '\n'
using namespace std;
typedef pair<int, int> pr;
typedef array<int, 3> tp;
int A[MAX], B[MAX];
double dp[MAX][MAX];
bool vst[MAX];
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(0), cout.tie(0);
int N, K, X, Y;
vector<int> arr_A, arr_B;
double res = INF, val;
cin >> N >> K;
for (int i = 1; i <= N; i++) {
cin >> A[i] >> B[i];
arr_A.push_back(i), arr_B.push_back(i);
}
sort(arr_A.begin(), arr_A.end(), [&](int a, int b) { return A[a] < A[b]; });
sort(arr_B.begin(), arr_B.end(), [&](int a, int b) { return (B[a] == -1 ? INF : B[a]) < (B[b] == -1 ? INF : B[b]); });
for (int i = 0; i <= K; i++) {
for (int j = 0; j <= N; j++)
for (int k = 0; k <= N; k++)
dp[j][k] = INF;
dp[0][0] = 0;
for (int j = 1; j <= K; j++) {
if (B[arr_B[j - 1]] == -1)
break;
dp[j][0] = dp[j - 1][0] + (double)B[arr_B[j - 1]] / j;
for (int k = 1; k <= j; k++) {
if (k != j)
dp[j][k] = min(dp[j - 1][k] + (double)B[arr_B[j - 1]] / (j - k), dp[j - 1][k - 1] + (double)A[arr_B[j - 1]] / (i + 1));
else
dp[j][k] = dp[j - 1][k - 1] + (double)A[arr_B[j - 1]] / (i + 1);
}
}
for (int j = 0; i + j <= K; j++) {
fill(vst, vst + N + 1, false);
val = dp[i + j][j];
for (int k = 0; k < i + j; k++)
vst[arr_B[k]] = true;
X = 0;
for (int k = 0; k < N && X < K - i - j; k++) {
if (!vst[arr_A[k]])
val += (double)A[arr_A[k]] / (i + 1), X++;
}
res = min(res, val);
}
}
cout << fixed << setprecision(10) << res << endl;
return 0;
}
