HDU 1009 FatMouse' Trade

import java.io.BufferedInputStream;
import java.util.Arrays;
import java.util.Scanner;

public class Main {
private static Scanner cin = new Scanner(new BufferedInputStream(System.in));
private static int n, m;
private static Node[] node;

public static void main(String[] args) {
double sum;
while (true) {
n = cin.nextInt();
m = cin.nextInt();
if (n + m == -2) {
break;
}
sum = 0;
node = new Node[m];
for (int i = 0; i < m; i++) {
node[i] = new Node(cin.nextInt(), cin.nextInt());
}
Arrays.sort(node);
for (int i = 0; i < m && n > 0; i++) {
if (n > node[i].y) {
n -= node[i].y;
sum += node[i].x;
} else {
sum += n * 1.0 / node[i].y * node[i].x;
n = 0;
}
}
System.out.printf("%.3f", sum);//直接加\n竟然PE了， 需要\r\n或者加上下面一行
System.out.println();
}
}
}

class Node implements Comparable<Node>{
int x, y;
double p;
public Node(int x, int y) {
this.x = x;
this.y = y;
setP();
}
void setP(){
p = x * 1.0 / y;
}
@Override
public int compareTo(Node node) {
return (p < node.p)? 1:-1;
}
}