# Write a function that takes a string
# replace every even position of characters in string
# with "@^-^@" and return the modified string

def modify_string(string):

# create a string variable for the result
# loop over the string
#    check if the index is even
#        concat @^-^@ to the result
#    otherwise
#        concat whatever the current letter to the result
# return the result

    result = ""
    for i in range(len(string)):    # i = 0, 1, 2, ...,
        if i % 2 == 0:
            result += "@^-^@"
        else:
            result += string[i]
    return result







print(modify_string("introduction to python"))