I have a problem with my code. It is my first assignment so maybe I'm doing something wrong with the return value. I really don't know. The program is written in C.

ASSIGNMENT:
If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.

Finish the solution so that it returns the sum of all the multiples of 3 or 5 below the number passed in.

Note: If the number is a multiple of both 3 and 5, only count it once.

CODE:

int solution(int number) {
// code here
int k;
scanf("%d", &k);
int a = 0;
int b = 0;
int c = 0;
int d = 0;
int i = 0;
int j = 0;
int sum = 0;
if(k % 3 != 0 && k % 5 == 0){
a = k / 3;
b = k / 5;
for(i=1; i<=a; i++){
c += 3 * i;
//printf("c u koracima = %d\n", c);
}
for(j=0; j<b; j++){
d += 5 * j;
//printf("d u koracima = %d\n", d);
}
sum = c + d;
}
else if(k % 3 != 0 && k % 5 != 0){
a = k / 3;
b = k / 5;
for(i=1; i<=a; i++){
c += 3 * i;
//printf("c u koracima = %d\n", c);
}
for(j=0; j<=b; j++){
d += 5 * j;
//printf("d u koracima = %d\n", d);
}
sum = c + d;
}
else if(k % 3 == 0 && k % 5 == 0){
a = k / 3;
b = k / 5;
for(i=0; i<a; i++){
c += 3 * i;
//printf("c u koracima = %d\n", c);
}
for(j=0; j<b; j++){
d += 5 * j;
//printf("d u koracima = %d\n", d);
}
sum = c + d;
}
else{
a = k / 3;
b = k / 5;
for(i=0; i<a; i++){
c += 3 * i;
//printf("c u koracima = %d\n", c);
}
for(j=1; j<=b; j++){
d += 5 * j;
//printf("d u koracima = %d\n", d);
}
sum = c + d;
}
//printf("a = %d\n", a);
//printf("b = %d\n", b);
//printf("c = %d\n", c);
//printf("d = %d\n", d);
//printf("sum = %d\n", sum);
return sum;
}

Would you be so kind and review my code?
What am I doing wrong?