import std.algorithm, std.array, std.container, std.range;
import std.string, std.conv;
import std.numeric, std.math, std.bigint, std.bitmanip, std.random;
import std.stdio, std.typecons;

void main()
{
  auto rd = readln.split.map!(to!int);
  auto k = rd[0], n = rd[1], f = rd[2];
  auto ai = readln.split.map!(to!int);

  auto r = k * n - ai.sum;
  writeln(r >= 0 ? r : -1);
}