[Tutor] please help - stuck for hours

[ADRIAN KELLY]

i have tried everything, i am trying to build in a loop to my 2 functions which worked fine up until my latest sorti.
please have a look if you can..............

def exchange(cash_in):    euro=1    dollar=1.35    base=50    if cash_in>base:        totalreturn=cash_in*dollar    else:        totalreturn=0    return totalreturn

def main():    amount=""    while amount<50:        print 'Sorry, cannot convert an amount under €50 '        amount = input('how much do you want to change:')    else:        total=exchange(amount)        print 'Your exchange comes to: ',total      main()
 
Traceback (most recent call last):  File "F:\VTOS ATHLONE\PYTHON_VTOS\foreign exchange\f_ex3.py", line 27, in <module>    main()  File "F:\VTOS ATHLONE\PYTHON_VTOS\foreign exchange\f_ex3.py", line 19, in main    total=exchange(amount)  File "F:\VTOS ATHLONE\PYTHON_VTOS\foreign exchange\f_ex3.py", line 7, in exchange    totalreturn=cash_in*dollarTypeError: can't multiply sequence by non-int of type 'float'>>> 
 
 		 	   		  
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