function Main(input) {
  const data = input.trim().split("\n")

  const first_line = data[0].split(" ")
  const K = parseInt(first_line[0]), N = parseInt(first_line[1]), F = parseInt(first_line[2])
  const second_line = data[1].split(" ")
  let sum = 0
  second_line.forEach(a => {
    sum += parseInt(a)
  })
  let result = K * N - sum
  if (result < 0) result = -1
  console.log(result)
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"))