EvenNumberSum_0915.cpp

#include <cstdlib>

bool isEven(int n)
{
    return n % 2 == 0;
}

// Note the computation order of operator
int computeSumToNumber(int n)
{
    return (n * (n + 1)) / 2;
}

int main(int argc, char* argv[])
{
    int n = -1;
    
    // Only even number is allowed
    while (!isEven(n)) scanf("%d", &n);
    
    printf("%d", computeSumToNumber(n / 2));
    
    return 0;
}