How to mark the Green Obelisk from the Center ?

So, i'm marking all 3 obelisks on the Center, and i can't mark the Green Obelisk. It's coords are 55 and 03 but when i input 55 and 03 in the map waypoint creator, it just tells me : Numeric numbers only (10:09) May not be exact, i translated it from the french version. To properly locate myself i need to mark this obelisk too !!! Help please !