# The if statement can detect if a value is wrong (less than zero in this case)...
# ...but the user could still enter bogus values the second time.

def Distance(X1,Y1,X2,Y2):
    DeltaX = X2 - X1
    DeltaY = Y2 - Y1
    return sqrt(DeltaX*DeltaX + DeltaY*DeltaY)

def Main():
    A1 = input("Enter X1 --- ")
    if (A1 < 0):
        print "This value is bogus"
        A1 = input("Enter X1 --- ")
        
    B1 = input("Enter Y1 --- ")
    if (B1 < 0):
        print "This value is bogus"
        B1 = input("Enter Y1 --- ")

    A2 = input("Enter X2 --- ")
    if (A2 < 0):
        print "This value is bogus"
        A2 = input("Enter X2 --- ")
        
    B2 = input("Enter Y2 --- ")
    if (B1 < 0):
        print "This value is bogus"
        B2 = input("Enter Y2 --- ")
        
    print Distance(A1,B1,A2,B2)
    return