#include <bits/stdc++.h>
using namespace std;
struct state {
int a, b;
};
const int MAX_N = 500;
const double INF = 1e9;
state states[MAX_N + 1];
double dp[2][MAX_N + 1][MAX_N + 1];
int main() {
int n, k;
cout << fixed << setprecision( 15 );
cin >> n >> k;
for ( int i = 1; i <= n; i++ )
cin >> states[i].a >> states[i].b;
sort( states + 1, states + n, []( state x, state y ) { return x.b < y.b; } );
double minCost = INF;
for ( int mask = 0; mask < (1 << n); mask++ ) {
double cost = 0;
int speakers = 1;
vector<int> rest;
bool ok = true;
for ( int i = 0; i < n; i++ ) {
if ( (mask >> i) & 1 ) {
if ( states[i + 1].b == -1 )
ok = false;
cost += (double)states[i + 1].b / speakers;
speakers++;
} else
rest.push_back( states[i + 1].a );
}
if ( speakers >= k + 1 || !ok )
continue;
sort( rest.begin(), rest.end() );
for ( int i = 0; i < k - speakers + 1; i++ )
cost += (double)rest[i] / speakers;
minCost = min( minCost, cost );
}
cout << minCost << "\n";
/*long double minCost = INF;
for ( int totalB = 0; totalB <= k; totalB++ ) {
bool side = false;
for ( int a = 0; a <= k; a++ ) {
for ( int b = 0; a + b <= k; b++ )
dp[side][a][b] = INF;
}
dp[side][0][0] = 0.0;
for ( int i = 1; i <= n; i++ ) {
side ^= 1;
for ( int a = 0; a <= k; a++ ) {
for ( int b = 0; b <= k - a; b++ )
dp[side][a][b] = dp[1 - side][a][b];
}
for ( int a = 0; a <= k; a++ ) {
for ( int b = 0; b <= k - a; b++ ) {
dp[side][a + 1][b] = min( dp[side][a + 1][b], dp[1 - side][a][b] + (long double)states[i].a / (totalB + 1) );
if ( states[i].b != -1 )
dp[side][a][b + 1] = min( dp[side][a][b + 1], dp[1 - side][a][b] + (long double)states[i].b / (b + 1) );
}
}
}
minCost = min( minCost, dp[side][k - totalB][totalB] );
}
cout << minCost << "\n";*/
return 0;
}
