挺有意思的題目，解題如下：

#include <stdio.h>

int main() {
    int ch;
    int braces;

    braces = 0;

    while ((ch = getchar()) != EOF) {
        if (ch == '{')
        {
            braces += 1;
        }

        if (ch == '}')
        {
            if (braces == 0)
            {
                printf("Extra closing brace!\n");
            } else {
                braces -= 1;
            }
        }
    }

    if (braces > 0)
    {
        printf("%d unmatched opening braces!\n", braces);
    }

    return 0;
}