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#include <iostream>
#include <vector>

using namespace std;

int main() {
    long h, w, n;
    cin >> h >> w >> n;

    vector<long> wymiary(n);
    for (int i = 0; i < n; i++) {
        cin >> wymiary[i];
    }

    long ilosc = 0;
    long obsz = 0;
    for (int i = n - 1; i >= 0; i--) {
        while (obsz < h * w) {
            obsz += wymiary[i] * wymiary[i];
            ilosc++;
        }
        if (obsz == h * w) {
            cout << ilosc << endl;
            return 0;
        }
        obsz -= wymiary[i] * wymiary[i];
        ilosc--;
    }

    cout << "-1" << endl;

    return 0;
}