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#include <cstdio>
#include <cstdlib>
using namespace std;

long long d[31];

struct filling_res {
    long long blocks;
    long long remainder;
};

filling_res fill(long long total_len, long long blocksize) {
    long long n_blocks = total_len / blocksize;
    long long r_blocks = total_len % blocksize;

    return filling_res{n_blocks, r_blocks};
}

long long fill_rec_aux(long long, long long, long long);

long long fill_rec(long long w, long long h, long long maxsquare) {
    if (w < h) {
        long long temp = w;
        w = h;
        h = temp;
    }
    // w >= h
    long long maxfit_idx = 0;
    for (long long i = maxsquare; i >= 0; --i) {
        if (d[i] <= h) {
            maxfit_idx = i;
            break;
        }
    }
    // printf("FILLING %lldx%lld with sq size %lld\n", w, h, d[maxfit_idx]);

    filling_res initrem = fill(h, d[maxfit_idx]);

    long long add_res = 0;
    if (initrem.remainder != 0) {
        add_res = fill_rec(w, initrem.remainder, maxfit_idx - 1);
    }

    return fill_rec_aux(w, h, maxfit_idx) + add_res;
}

long long fill_rec_aux(long long h, long long w, long long sq) {
    // printf("AUXING %lldx%lld with sq size %lld\n", h, w, d[sq]);
    // d[sq] is initial w
    long long rem = h;
    long long multiplier = w / d[sq];
    long long totalblocks = 0;
    while(rem != 0) {
        filling_res res = fill(rem, d[sq]);
        rem = res.remainder;
        totalblocks += res.blocks * multiplier;
        // printf("T: %lld, REM: %lld, S: %lld\n", totalblocks, rem, d[sq]);
        if (sq != 0) {
            // printf("SQ: %lld, DSQ: %lld, DSQ1: %lld\n", sq, d[sq], d[sq-1]);
            // printf("Multiplying multiplier by %lld\n", d[sq] / d[sq-1]);
            multiplier *= d[sq] / d[sq-1];
        }
        sq--;
    }
    // printf("AUXED %lld\n", totalblocks);
    return totalblocks;
}



int main() {
    long long h, w, n;
    scanf("%lld %lld", &h, &w);
    scanf("%lld", &n);
    for (long long i = 0; i < n; ++i) {
        scanf("%lld", &d[i]);
    }
    if (h % d[0] != 0 || w % d[0] != 0) {
        printf("-1\n");
        return 0;
    }

    printf("%lld\n", fill_rec(w, h, n-1));
    return 0;
}