#include <bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i,a,b) for (int i = a; i <= b; ++i)
#define pii pair<int,int>
#define fi first
#define se second
const int MAXN = 510;
pii a[MAXN];
int32_t main() {
ios_base::sync_with_stdio(0); cin.tie(nullptr);
int n,k; cin >> n >> k;
rep(i,1,n) {
cin >> a[i].se >> a[i].fi;
if (a[i].fi==-1) a[i].fi = 1e9;
}
sort(a+1, a+n+1);
double ans = 1e9;
rep(j,0,n) {
if (j > 0 && a[j].fi == 1e9) break;
double cur = 0;
int cnt = 1;
rep(i,1,n) {
if (i <= j) {
cur += (double)a[i].fi/cnt;
cnt++;
} else {
cur += (double)a[i].se/cnt;
}
}
ans = min(ans,cur);
}
cout << fixed << setprecision(10) << ans << '\n';
}
