#include <bits/stdc++.h>
using namespace std;
struct state {
int a, b;
};
const int MAX_N = 500;
const double INF = 1e6;
int n, k;
state states[MAX_N + 1];
double dp[2][MAX_N + 1][MAX_N + 1];
double solve( int totalB ) {
bool side = 0;
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; a + b <= k; b++ )
dp[side][a][b] = dp[1 - side][a][b];
}
for ( int a = 0; a <= k - totalB; a++ ) {
for ( int b = 0; a + b <= k && b <= totalB; b++ ) {
dp[side][a + 1][b] = min( dp[side][a + 1][b], dp[1 - side][a][b] + (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] + (double)states[i].b / (b + 1));
}
}
}
return dp[side][k - totalB][totalB];
}
int main() {
cin >> n >> k;
for ( int i = 1; i <= n; i++ )
cin >> states[i].a >> states[i].b;
cout << fixed << setprecision( 6 );
sort( states + 1, states + 1 + n, []( state x, state y ) { return x.b < y.b; } );
int l = 0, r = k + 1;
while ( r - l > 1 ) {
int mid = (l + r) / 2;
if ( solve( mid ) < solve( mid - 1 ) )
l = mid;
else
r = mid;
}
cout << solve( l ) << "\n";
return 0;
}
