Sum of two numbers --- with data validity and multiple query

Script

#!/usr/local/bin/python3.4
import sys

sum = 0

value = -1

while value == -1:
        message = 'Enter a number:  '
        try:
  value1 = float(input(message))
        except Exception:
  print("Enter a valid number")
  continue

message = 'Enter a second number:  '
try:
value2 = float(input(message))
except Exception:
  print("Enter a valid number")
  continue

sum =  value1 + value2

print('The sum of ', value1, 'and', value2, 'is', sum)
continue_message = "Do you want to continue(y/n): "
c = input(continue_message)
if (c == 'n') or  (c == 'N'):
value = 0
print("Thank you!")


Execution

Enter a number:  45
Enter a second number:  82
The sum of  45.0 and 82.0 is 127.0
Do you want to continue(y/n): y
Enter a number:  267
Enter a second number:  851
The sum of  267.0 and 851.0 is 1118.0
Do you want to continue(y/n): y
Enter a number:  thomas
Enter a valid number
Enter a number:  brian
Enter a valid number
Enter a number:  91
Enter a second number:  57
The sum of  91.0 and 57.0 is 148.0
Do you want to continue(y/n): n
Thank you!