I am new in Quantconnect and struggling with a basic algo . 

I am looking to script an intro algo  that  will sell a pair of currency when the closing of last price [1] > SMA50 [0] and price [0] < SMA50[0]

I use the rolling window for SMA but cant know how tu use the one for prices ( ie close prices)  

I used this for SMA and it worked but while using for closes i get an error

SMA50 = SMA(Symbol, SlowMAPeriod, AlgoResolution);
SMAValues = new RollingWindow<decimal>(2);



I tired this for price closes but it doenst work > 

 private RollingWindow<TradeBar> CloseValues; // i tried using tradeBar and Quotebar


CloseValues = new RollingWindow<decimal>(2);

How can i retrieve the close of t-1?

Thank you for your help