The first one is X, then the second one would be X+1 ( adding one, makes it consecutive).
Now you have X + X+1 = 667
Simplify:
2x +1 = 667
Subtract 1 from each side:
2x = 666
Divide both sides by 2:
x = 666 /2
x = 333
The first marker is 333
The second marker = 333 +1 = 334
