// Autor: Nicolas Wochnik
#include <iostream>
#include <vector>
int main() {
  std::ios_base::sync_with_stdio(false);
  std::cin.tie(nullptr);
  long long height, width;
  std::cin >> height >> width;
  size_t blocks_size;
  std::cin >> blocks_size;
  // if (blocks_size > 30) {
  //   return 1;
  // }
  std::vector<long long> blocks(blocks_size);
  for (auto &block : blocks) {
    std::cin >> block;
  }
  std::vector<long long> needed_blocks(blocks.size());
  for (size_t i = 0; i < blocks.size(); ++i) {
    long long remaining = height;
    long long ptr = i;
    long long needed = 0;
    while (ptr >= 0) {
      long long amount = remaining / blocks[ptr];
      remaining -= blocks[ptr] * amount;
      needed += (blocks[i] / blocks[ptr]) * amount;
      ptr--;
    }
    if (remaining != 0) {
      std::cout << "-1\n";
      return 0;
    }
    needed_blocks[i] = needed;
  }
  // for (auto var : needed_blocks) {
  //   std::cout << var << " ";
  // }
  // std::cout << "\n";
  long long remaining = width;
  long long ptr = blocks.size() - 1;
  long long needed = 0;
  while (ptr >= 0) {
    long long amount = remaining / blocks[ptr];
    remaining -= blocks[ptr] * amount;
    needed += needed_blocks[ptr] * amount;
    ptr--;
  }
  if (remaining != 0) {
    std::cout << "-1\n";
    return 0;
  }
  std::cout << needed << "\n";
  return 0;
}
        1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70  | // Autor: Nicolas Wochnik #include <iostream> #include <vector> int main() { std::ios_base::sync_with_stdio(false); std::cin.tie(nullptr); long long height, width; std::cin >> height >> width; size_t blocks_size; std::cin >> blocks_size; // if (blocks_size > 30) { // return 1; // } std::vector<long long> blocks(blocks_size); for (auto &block : blocks) { std::cin >> block; } std::vector<long long> needed_blocks(blocks.size()); for (size_t i = 0; i < blocks.size(); ++i) { long long remaining = height; long long ptr = i; long long needed = 0; while (ptr >= 0) { long long amount = remaining / blocks[ptr]; remaining -= blocks[ptr] * amount; needed += (blocks[i] / blocks[ptr]) * amount; ptr--; } if (remaining != 0) { std::cout << "-1\n"; return 0; } needed_blocks[i] = needed; } // for (auto var : needed_blocks) { // std::cout << var << " "; // } // std::cout << "\n"; long long remaining = width; long long ptr = blocks.size() - 1; long long needed = 0; while (ptr >= 0) { long long amount = remaining / blocks[ptr]; remaining -= blocks[ptr] * amount; needed += needed_blocks[ptr] * amount; ptr--; } if (remaining != 0) { std::cout << "-1\n"; return 0; } std::cout << needed << "\n"; return 0; }  | 
            
        
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