#include <stdio.h>

int getAmari(unsigned int num_of_bean, unsigned int *family, unsigned int size) {
    int sum_age = 0;
    int i = 0;
    for(i = 0; i < size; i++) {
        sum_age += *(family + i);
    }
    if(sum_age > num_of_bean) {
        return -1;
    }
    return num_of_bean - sum_age;
}

int main(void) {
    unsigned int K = 0;
    unsigned int N = 0;
    unsigned int F = 0;

    scanf("%d %d %d", &K, &N, &F);

    unsigned int family[101] = {};
    unsigned int itr = 0;
    for (itr = 0;itr < F; itr++) {
        scanf("%d", &family[itr]);
    }

    printf("%d\n", getAmari(K*N, family, F));
    return 0;
}