offsetof() theory and practice header stddef.h

#include<stdio.h>
#include<stddef.h>
struct estructura{
    int memberinstructure; //0
    char array[20]; //0 + sizeof(int) = 0 + 4    "4 because sizeof(int) = 4 in memberinstructure"
    int end; //0 + sizeof(int) + (20 * sizeof(char)) = 0 + 4 + (20 * 1) "20 because sizeof(char) = 1 and array = 20"     "20 * 1 = 20"
    //But the offsetof never give total size from struct.
    //Total size = 0 + 4 + (20 * 1) + 4 = 28 "The ultimate 4 because int end == sizeof(int) = 4"
};
void main(){
    printf("Size: %d\n",offsetof(struct estructura,end));
    //offsetof() theory--------------------------------------------
    int operation1 = 0 + sizeof(int);
    int operation2 = operation1 + (20 * sizeof(char));
    //-------------------------------------------------------------
    printf("offsetof() Theory Operation =  %d\n",operation2);
    printf("Is %d == %d ??",offsetof(struct estructura,end),operation2);
    if (offsetof(struct estructura,end) == operation2){
        printf("True\n\n");
    }
    //-------------------------------------------------------------
}