main :: IO ()
main = do
    -- Read the first line and parse K, N, and F
    firstLine <- getLine
    let [k, n, f] = map read (words firstLine) :: [Int]
    
    -- Read the second line and parse the ages
    secondLine <- getLine
    let ages = map read (words secondLine) :: [Int]
    
    -- Calculate total beans available and total beans required
    let totalBeans = k * n
    let requiredBeans = sum ages
    
    -- Calculate the remaining beans
    let leftover = totalBeans - requiredBeans
    
    -- Output -1 if there are not enough beans, otherwise output the leftover
    if leftover < 0
        then print (-1)
        else print leftover