#include <stdio.h>

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]);
    }
    unsigned int sum_age = 0;
    for(itr = 0; itr < F; itr++) {
        sum_age += *(family + itr);
        if(sum_age > K*N) {
            printf("%d\n", -1);
            return 0;
        }
    }
    printf("%d\n", K*N - sum_age);
    return 0;
}