#include <bits/stdc++.h>
#define ar array
//#define int long long
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
const int mod = 1e9 + 7;
const ll inf = 1e18;
const int N = 1e5 + 5;
signed main() {
ios_base::sync_with_stdio(false);
cout.tie(0); cin.tie(0);
int n, x, cnt=0; cin >> n >> x;
vector<ar<ll, 2> > a(n+1);
for(int i=1; i<=n; i++) {
cin >> a[i][1] >> a[i][0];
if(a[i][0] != -1) cnt++;
}
sort(a.begin()+1, a.end());
double ans = 1e18, dp[n+1][x+1][x+1];
for(int c=0; c<=min(x, cnt); 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++) {
for(int k=0; k<=min(j, c); k++) {
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] != -1) {
dp[i][j][k] = min(dp[i][j][k], dp[i-1][j-1][k-1] + (double)a[i][0] / k);
}
}
}
}
ans = min(ans, dp[n][x][c]);
}
cout << setprecision(3) << fixed << ans << '\n';
}
