#Python 3, 70 Chars

Loops through, finding the square of the difference and then the root of the sum:

a=input() b=input() x=sum([(a[i]-b[i])**2 for i in range(len(a))])**.5 

Python 3, 70 Chars

Loops through, finding the square of the difference and then the root of the sum:

a=input() b=input() x=sum([(a[i]-b[i])**2 for i in range(len(a))])**.5 

#Python 3, 71 Chars

Loops through, finding the square of the difference and then the root of the sum:

a=input() b=input() x=sum([(a[i]-b[i])**2 for i in range(len(a))])**0.5 

#Python 3, 70 Chars

Loops through, finding the square of the difference and then the root of the sum:

a=input() b=input() x=sum([(a[i]-b[i])**2 for i in range(len(a))])**.5