#include <bits/stdc++.h>
using ll = long long;
// #define int ll
using namespace std;
#define sz(x) (int)(x).size()
#define foru(i, l, r) for(int i = l; i <= r; i++)
#define ford(i, l, r) for(int i = l; i >= r; i--)
#define fi first
#define se second
#define mod 998244353
#define db(x) cerr << __LINE__ << " " << #x << " " << x << "\n"
using vi = vector<int>;
using pi = pair<int, int>;
const ll N = 505;
const ll inf = 1e9;
ll n, k;
vector<pi> state(N);
double dp[N][N];
void solve(){
cin >> n >> k;
state.resize(n);
foru(i, 0, n - 1){
cin >> state[i].fi >> state[i].se;
if(state[i].se == -1){
state[i].se = inf;
}
}
sort(state.begin(), state.end(), [&](pi a, pi b){
return a.se < b.se;
});
double res = inf;
foru(i, 0, k){
dp[0][0] = 0;
foru(j, 1, k - i) dp[0][j] = inf;
foru(j, 1, n){
foru(f, 0, k - i){
ll zz = j - f;
auto kap = ((1 <= zz and zz <= i) ? 1.0 * state[j - 1].se / zz : 0);
dp[j][f] = dp[j - 1][f] + kap;
if(f){
dp[j][f] = min(dp[j][f], dp[j - 1][f - 1] + 1.0 * state[j - 1].fi / (i + 1));
}
}
}
res = min(res, dp[n][k - i]);
}
cout << fixed << setprecision(100) << res;
}
signed main(){
ios_base::sync_with_stdio(0); cin.tie(0);
int t = 1;
while(t--){
solve();
}
}
