Why doesn't it check if the key is in the dictionary?

Question

C++ / C#

here is my dictionary

private Dictionary<int, int> CellsMoove = new Dictionary<int, int>();

I check if there is no such value, then I enter, if there is, then I replace

int Num1 = 1;
int Num2 = 2;
if(!CellsMoove.ContainsKey(Num1)) CellsActive.Add(Num1, Num2); else CellsMoove[Num1] = Num2;

but it gives me an error

Argument 2: cannot convert from 'int' to 'bool'

Is ContainsKey supposed to return true or false , or what's wrong?