Can I convert a structure string to an integer?

int main()
{
    void pid_sort(struct id[], int);
    void pid_print(struct id[], int);
    struct student stu[3];
    int count;
    int i;

    printf ("Enter 3 student information");

    for (i = 0; i < 3; i++)
    {
        printf ("Name: ");
        scanf_s("%s", stu[i].name, sizeof(stu[i].name));
        printf ("Department: ");
        scanf_s("%s", stu[i].subject, sizeof(stu[i].subject));
        printf ("resident registration number: ");
        scanf_s("%s", stu[i].pid, sizeof(stu[i].pid));


        stu[i].S_Y = stu[i].pid[6];
        stu[i].year = stu[i].pid[0]*10 + stu[i].pid[1];
        stu[i].month = stu[i].pid[2]*10 + stu[i].pid[3];
        stu[i].day = stu[i].pid[4]*10 + stu[i].pid[5];