请用c语言写代码，要求如下：已知一个正整数序列，个数未知，但至少有一个元素，你的任务是建立一个单链表，并使用该链表存储这个正整数序列，然后统计这个序列中元素的最大值与最小值，计算序列全部元素之和。正整数的输入用-1作为结束标志，注意-1不算这个正整数序列中的元素（不要统计-1）。

#include <stdio.h>
#include <stdlib.h>

struct Node {
    int val;
    struct Node* next;
};

int main() {
    struct Node *head = NULL, *curr = NULL;
    int num, max = -1, min = -1, sum = 0;

    while (1) {
        printf("Enter a positive integer (-1 to end): ");
        scanf("%d", &num);

        if (num == -1) {
            break;
        }

        if (num < 0) {
            printf("Invalid input. Please enter a positive integer.\n");
            continue;
        }

        if (curr == NULL) {
            head = (struct Node*) malloc(sizeof(struct Node));
            curr = head;
        } else {
            curr->next = (struct Node*) malloc(sizeof(struct Node));
            curr = curr->next;
        }

        curr->val = num;
        curr->next = NULL;

        if (max == -1 || num > max) {
            max = num;
        }

        if (min == -1 || num < min) {
            min = num;
        }

        sum += num;
    }

    if (head == NULL) {
        printf("Error: empty sequence.\n");
        return 1;
    }

    printf("Max: %d\n", max);
    printf("Min: %d\n", min);
    printf("Sum: %d\n", sum);

    return 0;
}