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#include <algorithm>
#include <cinttypes>
#include <cmath>
#include <cstdlib>
#include <cstdio>
#include <cstdint>
#include <deque>
#include <initializer_list>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using std::deque;
using std::initializer_list;
using std::make_pair;
using std::map;
using std::multiset;
using std::pair;
using std::priority_queue;
using std::reverse;
using std::set;
using std::sort;
using std::string;
using std::swap;
using std::unordered_map;
using std::unordered_set;
using std::vector;

typedef int8_t     int8;
typedef int16_t    int16;
typedef int32_t    int32;
typedef int64_t    int64;
typedef __int128_t int128;

template <typename T>
using priority_queue_max = std::priority_queue<T>;

template <typename T>
using priority_queue_min = std::priority_queue<T, std::vector<T>, std::greater<T>>;

const int32 oo = 1e9;
const int64 ooo = 1e18;

#define scan(args...) [&]{ return scanf(args); }();
#define elements(a) ((int32)(a).size())
#define TESTS  int32 _t; scan("%" SCNd32, &_t); while (_t--)

inline int64 castformorebits(int32 a)
{
    return (int64)a;
}

inline int128 castformorebits(int64 a)
{
    return (int128)a;
}

template <class INT>
inline INT abs(INT a)
{
    if (a >= 0) return a;
    return -a;
}

template <typename T>
T minimum(std::initializer_list<T>&& l)
{
    T answer = *(l.begin());

    for (auto a : l)
        if (a < answer)
            answer = a;

    return answer;
}

template <typename T>
T minimum(const vector<T>& l)
{
    T answer = *(l.begin());

    for (auto a : l)
        if (a < answer)
            answer = a;

    return answer;
}

template <typename T>
T maximum(std::initializer_list<T>&& l)
{
    T answer = *(l.begin());

    for (auto a : l)
        if (a > answer)
            answer = a;

    return answer;
}

template <typename T>
T maximum(const vector<T>& l)
{
    T answer = *(l.begin());

    for (auto a : l)
        if (a > answer)
            answer = a;

    return answer;
}

template <typename T>
inline void sort(T &data)
{
    sort(data.begin(), data.end());
}

template <typename T>
inline void reverse(T &data)
{
    reverse(data.begin(), data.end());
}


inline void read(int32& val)
{
    scan("%" SCNd32, &val);
}

inline void read(int64& val)
{
    scan("%" SCNd64, &val);
}

inline void read(double& val)
{
    scan("%lf", &val);
}

inline void read(string& val, int length = 1e6)
{
    char tmp[length+1];

    scan("%s", tmp);

    val = tmp;
}

template <class S, class T>
inline void read(S& a, T& b)
{
    read(a);
    read(b);
}

template <class S>
inline void read(vector<S> &v, int n)
{
    v.resize(n);
    for (int i = 0; i < n; i++)
        read(v[i]);
}

template <class S, class T, class U>
inline void read(S& a, T& b, U& c)
{
    read(a);
    read(b);
    read(c);
}

template <class S, class T, class U, class V>
inline void read(S& a, T& b, U& c, V& d)
{
    read(a);
    read(b);
    read(c);
    read(d);
}

template <class S, class T, class U, class V, class W>
inline void read(S& a, T& b, U& c, V& d, W& e)
{
    read(a);
    read(b);
    read(c);
    read(d);
    read(e);
}

inline void write(int32 val)
{
    printf("%" PRId32 " ", val);
}

inline void write(int64 val)
{
    printf("%" PRId64 " ", val);
}

inline void write(double val)
{
    printf("%lf ", val);
}

inline void write(const string& val)
{
    printf("%s ", val.c_str());
}

template<class S>
inline void write(const vector<S>& val)
{
    for (S el : val)
        write(el);
}

template<class S>
inline void writeln(const S& a)
{
    write(a);
    printf("\n");
}

template<class S, class T>
inline void writeln(const S& a, const T& b)
{
    write(a);
    write(b);
    printf("\n");
}

template<class S, class T, class U>
inline void writeln(const S& a, const T& b, const U& c)
{
    write(a);
    write(b);
    write(c);
    printf("\n");
}

template<class S, class T, class U, class V>
inline void writeln(const S& a, const T& b, const U& c, const V& d)
{
    write(a);
    write(b);
    write(c);
    write(d);
    printf("\n");
}

template<class S, class T, class U, class V, class W>
inline void writeln(const S& a, const T& b, const U& c, const V& d, const W& e)
{
    write(a);
    write(b);
    write(c);
    write(d);
    write(e);
    printf("\n");
}

#ifdef PCL_DEBUG
    #define DBG_RED "\033[1;31m"
    #define DBG_YELLOW "\033[1;33m"
    #define DBG_BLUE "\033[1;34m"
    #define DBG_NC "\033[0m"

    #define debug(var) { printf("%s%s%s = ", DBG_RED, #var, DBG_NC); writeln(var); }
    #define break_point(str) { printf("%s%s%s\n", DBG_YELLOW, str, DBG_NC); }
#else
    #define debug(...)
    #define break_point(...)
#endif

int main()
{
    int h, w, n;
    vector<int> a;
    int64 ans = 0;

    read(h, w, n);
    read(a, n);

    if (h % a[0] != 0 or w % a[0] != 0)
    {
        writeln("-1");
        return 0;
    }

    h /= a[0];
    w /= a[0];

    ans = (int64)h * w;

    for (int i = 1; i < n; i++)
    {
        int real_a = a[i] / a[i - 1];

        int new_h = h / real_a;
        int new_w = w / real_a;

        ans -= (int64)new_h * new_w * real_a * real_a;
        ans += (int64)new_h * new_w;

        h = new_h;
        w = new_w;
    }

    writeln(ans);

    return 0;
}