## Problem statement

The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17. Find the sum of all the primes below two million.

## Thoughts

I think this just needs a brute-force approach. I did find this interesting formula on Quora while searching for alternatives, but it makes the solution pretty complicated, and I think it may even take longer than the brute force approach.

## Solution

Prime sieve up to 2 million, sum all the found primes on the way.

```
#include <stdio.h>
#include <math.h>
#include <stdbool.h>
#define LIMIT 2000000
bool primes[LIMIT];
int main()
{
/* Assume every integer is prime until we know otherwise */
for (int i = 0; i < LIMIT; ++i) {
primes[i] = true;
}
primes[0] = false; /* 0 is not a prime */
primes[1] = false; /* 1 is not a prime */
double sum = 0;
for (int i = 2; i < LIMIT + 1; ++i) {
if (primes[i]) {
sum += i;
for (int j = i + i; j < LIMIT; j += i) {
primes[j] = false;
}
}
}
printf("%f", sum);
}
```