#include <stdio.h>
#include <limits.h>
#include <stdlib.h>
#include <math.h>

int main() {
    FILE *fin = fopen("input.txt", "r");
    FILE *fout = fopen("output.txt", "w");

    int n, m, x, k;
    fscanf(fin, "%d%d%d%d", &n, &m, &x, &k);

    int  assump, porch, floor;
    int *house = (int *) calloc(n + 1, sizeof(int));
    for (int i = 1; i < k + 1; i++) {
        fscanf(fin, "%d", &assump);
        int(porch) = (x-1)/(m*assump)+1;
        int(floor) = ((x-(porch-1)*m*assump)-1)/assump+1;
        if ((porch - 1) <= n) {
            if (floor > house[porch])
                house[porch] = floor;
        }
    }
        int sum = 0;
        for (int i = 0; i < n + 1; i++) {
            sum += house[i];
    }
    if (sum==0) fprintf(fout,"%s","-1");
    else fprintf(fout,"%d",sum);

    fclose(fin);
    fclose(fout);

    return 0;
}