let inpt = readLine()!.split(separator: " ").map{Int($0)!}

let mameCount = inpt[0]*inpt[1]
let kazokuCount = inpt[2]

let kazoku = readLine()!.split(separator: " ").map{Int($0)!}.reduce(0){$0+$1}

let ans = max(-1, mameCount-kazoku)
print(ans)