题面

核心思想

1. 折扣价不能大于原价

2. 原价才能参与满家

3. 原价、折扣价和满减的价格都必须是正实数

4. 格式化输出

代码

import java.util.*;
public class Main {
    public static void main(String[] args) {
        final long MOD = (long) (1e9 + 7);
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();

        boolean flag = true;
        double sum = 0;
        double cur = 0;
        while(n-- > 0){
            double a = scanner.nextDouble();
            double b = scanner.nextDouble();

            if(b > a || a <= 0 || b <= 0){
                flag = false;
            }
            sum = sum + a;
            cur = cur + b;
        }
        double a = scanner.nextDouble();
        double b = scanner.nextDouble();
        if(b > a || a <= 0 || b <= 0)
            flag = false;

        if(flag){
            if(sum > a)
                sum -= b;
            cur = Math.min(cur, sum);
            System.out.printf("%.2f\n", cur);
        }

        if(!flag)
            System.out.println("error");
    }
}