Fibonacci Encryptor

def main():
    startAlgorithm()

def fibonacciSeq(n):
    a = 0
    b = 1
    for i in range(0, n - 2):
        c = a + b
        a = b
        b = c
    return c

characters = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M',
                'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z']

def startAlgorithm():
    while True:

        userInput = input("Enter the nth number Fibonacci sequence for the key then ; and then the word you want encoded: ")

        output = ""

        if userInput < 0:
            break

        n = int(userInput[0:userInput.find(';')])

        word = userInput[userInput.find(';') + 1:]

        for i in range(0, len(word)):
            output += characters[(characters.index(word[i].upper())
                                    + fibonacciSeq(n + i)) % 26]
        print(output)

main()