Please review my solution to Project Euler program 1

Can someone give me suggestions on how to improve this program?

The objective is to find the sum of all the multiples of 3 or 5 below 1000.

#include 

main()
{
    int S, s3,s2,S1;
    int a=333,b=3,c=999, i=5,j=995, k=199, m = 15, n= 66, o= 990;
    S1= a*(b+c)/2;
    s2= k*(i+j)/2;
    s3= n*(m+o)/2;
    S= S1 + s2 - s3;
    printf("sum is:%d",S);
    return 0;
}

You certainly got the point of the problem. You applied Inclusion-Exclusion principle. Your code works well too. Most of the operations you use are constant time operations and can't be optimized further. The only problem is code formatting. Every coder develops a coding style that suites him or herself, but when you code, always remember that your code can fall into someone else's hand one day and you don't want the person to suffer before understanding your code.

• A comment at the beginning of the code will be good. Not just for a third party reader, but you. You may open the code in another decade, you don't wanna have to go to project Euler's site before you understand what your code does.

• Naming variables, your variables name should reflect its meaning. And you initialized way too many variables on the same line. Funny enough those variables are pointless. You can just use their values immediately in S1, S2 and S3.

• Spacing also is important, not for the compiler of course but for you or whoever is reading this code.

This is how I would have done it;

#include

/* Function to find the sum of the multiples
of a OR b between 1 and N, excluding N */ 

int findSum(int a, int b, int N){
    N--;
    int a_n = (N/a)*a,
        b_n = (N/b)*b,
        ab_n = (N/(a*b))*a*b,
        sum_a = ((N/a) * (a + a_n))/2,
        sum_b = ((N/b) * (b + b_n))/2,
        sum_ab = ((N/(a*b)) * (a*b + ab_n))/2;
    return sum_a + sum_b - sum_ab;
}

int main(){
    int a = 3, b = 5, N = 1000;
    printf("%d", findSum(a, b, N));
    return 0;
}

StackExchange Code Review Q#31051, answer score: 2