#include <stdio.h>

int main() {
    int k, n, f;
    scanf("%d %d %d", &k, &n, &f);
    int sum = k*n;
    int tmp;
    for (int i=0; i<f; i++) {
        scanf("%d", &tmp);
        sum-=tmp;
    }
    printf("%d\n",(sum<0)?-1:sum);
    return 0;
}