Time Series, exponential smoothing--I need help!

Seems a really simple problem but it's bugging me.

I'm using the exponential smmothing formula:

NthForecast=k*(N-1)thY + (1-k)(N-1)thForecast, 0<k<1, where (N-1)thY is the (N-1)thY value, and Forecast is the forecast of a y value.

I can't work out what the hell I'm to do if I'm forecasting the Nth value, and I have the (N-1)thY value. What's the (N-1)th forecast to be taken as in the formula, seeing as I already have that value and don't want to forecast it?
If I make it equal to the (N-1)thY, then I just get: NthForecast=(N-1)thY

God I'm confused:confused:
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