let inpA = readLine()!
let inpB = readLine()!

let inpAs = inpA.split(separator: " ").map{Int($0)!}
let ages = inpB.split(separator: " ").map{Int($0)!}.reduce(0, +)

let beans = inpAs[0] * inpAs[1]
let rem = beans - ages

print(rem > 0 ? rem : -1)