prob_1

def sol(n):
    numbers = [0]*n

    char = 0 #store 8 bits here
    c = 0 #counter for converting from binary to decimal
    
    toreturn = "" #string to be returned
    
    for i in range(3,n,2):
        
        if numbers[i] != 1: #prime number
            char += 2 ** (7 - c) #convert it to decimal based on its position

            #delete all multiples of that prime number
            for j in range(i,n,i):
                numbers[j] = 1

        #char reached 8 bits: combine them to 1 byte, add it to toretrun, and reset char and c
        if (7 - c) == 0:
            toreturn += chr(char)
            char = 0
            c = -1
        c += 1 #update position
    return toreturn