#include <bits/stdc++.h>
using namespace std;

struct Scanner {
public:

  explicit Scanner(FILE *fp) : fp(fp) {}

  template< typename T, typename... E >
  void read(T &t, E &... e) {
    read_single(t);
    read(e...);
  }

private:
  static constexpr size_t line_size = 1 << 16;
  static constexpr size_t int_digits = 20;
  char line[line_size + 1] = {};
  FILE *fp = nullptr;
  char *st = line;
  char *ed = line;

  void read() {}

  static inline bool is_space(char c) {
    return c <= ' ';
  }

  void reread() {
    ptrdiff_t len = ed - st;
    memmove(line, st, len);
    char *tmp = line + len;
    ed = tmp + fread(tmp, 1, line_size - len, fp);
    *ed = 0;
    st = line;
  }

  void skip_space() {
    while(true) {
      if(st == ed) reread();
      while(*st && is_space(*st)) ++st;
      if(st != ed) return;
    }
  }

  template< typename T, enable_if_t< is_integral< T >::value, int > = 0 >
  void read_single(T &s) {
    skip_space();
    if(st + int_digits >= ed) reread();
    bool neg = false;
    if(is_signed< T >::value && *st == '-') {
      neg = true;
      ++st;
    }
    typename make_unsigned< T >::type y = *st++ - '0';
    while(*st >= '0') {
      y = 10 * y + *st++ - '0';
    }
    s = (neg ? -y : y);
  }

  template< typename T, enable_if_t< is_same< T, string >::value, int > = 0 >
  void read_single(T &s) {
    s = "";
    skip_space();
    while(true) {
      char *base = st;
      while(*st && !is_space(*st)) ++st;
      s += string(base, st);
      if(st != ed) return;
      reread();
    }
  }

  template< typename T >
  void read_single(vector< T > &s) {
    for(auto &d : s) read(d);
  }
};

struct Printer {
public:
  explicit Printer(FILE *fp) : fp(fp) {}

  ~Printer() { flush(); }

  template< bool f = false, typename T, typename... E >
  void write(const T &t, const E &... e) {
    if(f) write_single(' ');
    write_single(t);
    write< true >(e...);
  }

  template< typename... T >
  void writeln(const T &...t) {
    write(t...);
    write_single('\n');
  }

  void flush() {
    fwrite(line, 1, st - line, fp);
    st = line;
  }

private:
  FILE *fp = nullptr;
  static constexpr size_t line_size = 1 << 16;
  static constexpr size_t int_digits = 20;
  char line[line_size + 1] = {};
  char small[32] = {};
  char *st = line;

  template< bool f = false >
  void write() {}

  void write_single(const char &t) {
    if(st + 1 >= line + line_size) flush();
    *st++ = t;
  }

  template< typename T, enable_if_t< is_integral< T >::value, int > = 0 >
  void write_single(T s) {
    if(st + int_digits >= line + line_size) flush();
    if(s == 0) {
      write_single('0');
      return;
    }
    if(s < 0) {
      write_single('-');
      s = -s;
    }
    char *mp = small + sizeof(small);
    typename make_unsigned< T >::type y = s;
    size_t len = 0;
    while(y > 0) {
      *--mp = y % 10 + '0';
      y /= 10;
      ++len;
    }
    memmove(st, mp, len);
    st += len;
  }

  void write_single(const string &s) {
    for(auto &c : s) write_single(c);
  }

  void write_single(const char *s) {
    while(*s != 0) write_single(*s++);
  }

  template< typename T >
  void write_single(const vector< T > &s) {
    for(size_t i = 0; i < s.size(); i++) {
      if(i) write_single(' ');
      write_single(s[i]);
    }
  }
};

int main() {
  Scanner in(stdin);
  Printer out(stdout);

  int K, N, F;
  in.read(K, N, F);
  int b = 0;
  for (int i = 0; i < F; i++) {
    int A;
    in.read(A);
    b += A;
  }
  out.writeln(max(-1, K * N - b));
  return 0;
}