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func countPrimes(n int) int {
    primes := make([]bool, n)
    for i := 2; i < n; i++ {
        if !primes[i] {
            for j := i*2; j < n; j += i {
                primes[j] = true
            }
        }
    }
    sum := 0
    for i := 2; i < n; i++ {
        if !primes[i] {
            sum++
        }
    }
    return sum
    
}