can we add a new element to a structure in c code example

Example 1: struct in C

#include<stdio.h>
#include<string.h>

struct Student{
long int rollNo;
int age;
char name[50];
int totalMarks;
};

int main(){

    struct Student s1;
    //accessing members of the struct
    s1.rollNo = 1232234643;
    s1.age = 10;
    char s[]="Jonathan";
    strcpy(s1.name,s);
    s1.totalMarks = 450;

    printf("RollNumber: %d \nName: %s \nTotal Marks: %d",s1.rollNo,s1.name,s1.totalMarks);

return 0;
}

Example 2: declare structure in c

struct num{
 int a ;
 int b;
};

int main()
{
    struct num n;
    //accessing the elements inside struct
    n.a=10;
    n.b=20;
}