#include <bits/stdc++.h>
#define ALL(X) begin(X), end(X)
using i64 = long long;
using namespace std;
template <class T>
using vec = vector<T>;
template <class T>
istream& operator>>(istream& is, vec<T>& X) {
for (auto& x : X) {
is >> x;
}
return is;
}
template <class T>
constexpr T infty = 0;
template <>
constexpr int infty<int> = 1'000'000'000;
template <>
constexpr i64 infty<i64> = 1'000'000'000'000'000'000;
using ld = double;
constexpr ld inf = 1e308;
int main() {
cin.tie(nullptr)->sync_with_stdio(false);
int N, K; cin >> N >> K;
vec<pair<int, int>> BA(N);
for (auto& [b, a] : BA) {
cin >> a >> b;
if (b == -1) {
b = 777771449;
}
}
sort(ALL(BA));
vec<vec<ld>> dp(K + 1, vec<ld>(K + 1, inf));
auto calc = [&](int l) -> ld {
for (int i = 0; i <= l; i++) {
for (int j = 0; j <= K; j++) {
dp[i][j] = inf;
}
}
dp[0][0] = 0;
for (int i = 0; i < N; i++) {
auto [b, a] = BA[i];
for (int j = l; j >= 0; j--) {
if (j == l) {
for (int k = K; k > 0; k--) {
dp[j][k] = min(dp[j][k], dp[j][k - 1] + (ld)a / (l + 1));
if (j > 0 && b != 777771449) {
dp[j][k] = min(dp[j][k], dp[j - 1][k - 1] + (ld)b / j);
}
}
}
else if (i < K) {
dp[j][i + 1] = min(dp[j][i + 1], dp[j][i] + (ld)a / (l + 1));
if (j > 0 && b != 777771449) {
dp[j][i + 1] = min(dp[j][i + 1], dp[j - 1][i] + (ld)b / j);
}
dp[j][i] = inf;
}
}
}
return dp[l][K];
};
cout << fixed << setprecision(15);
ld ans{inf};
for (int i = 0; i <= K; i++) {
ans = min(ans, calc(i));
}
cout << ans << '\n';
}
