#include <stdio.h>

int main(void)
{
    int k, n, f;
    int sum;
    int ans;
    int buf;
    int i;

    scanf("%d", &k);
    scanf("%d", &n);
    scanf("%d", &f);

    sum = 0;
    for (i = 0; i < f; i++){
        scanf("%d", &buf);
        sum += buf;
    }

    if (k * n >= sum){
        printf("%d\n", k * n - sum);
    }
    else {
         printf("-1\n");
    }

    return 0;
}