mars_exploration.rb

# Sami’s spaceship crashed on Mars! She sends sequential SOS messages to Earth for help.
# Letters in some of the SOS messages are altered by cosmic radiation during transmission. Given the signal
# received by Earth as a string, S , determine how many letters of Sami’s SOS have been changed by radiation.

# Input Format
# There is one line of input: a single string, S .
# Note: As the original message is just SOS repeated n times, S ’s length will be a multiple of 3 .

# Constraints
# 1 ≤ |S| ≤ 99
# |S| % 3 = 0

# S will contain only uppercase English letters

# Output Format

# Print the number of letters in Sami’s message that were altered by cosmic radiation.

# Sample Input 0
# SOSSPSSQSSOR
# Sample Output 0
# 3

# Sample Input 1
# SOSSOT
# Sample Output 1
# 1
  
puts "give a sos messgae : "

sos_msg = gets.strip.upcase

# group the message with 3 letters and it will return an array
altered_msg = sos_msg.scan(/.{3}/)

changed_letter = 0

altered_msg.each do |msg|
	if msg != "SOS"
		changed_letter = changed_letter + 1
	end
end

puts "Number of changed letter is #{changed_letter}"