#include <stdio.h>

int main(){
    int K, N, F;
    int family[100];
    int i, sum = 0;

    scanf("%d %d %d\n", &K, &N, &F);
    for(i = 0; i < F; i++){
        scanf("%d", &family[i]);
        sum += family[i];
    }

    if(sum <= K*N){
        printf("%d\n", K*N - sum);
    }else{
        printf("-1\n");
    }

    return 0;
}