#include <stdio.h>
int main(){
    //input
    int K,N,F;
    scanf("%d %d %d",&K,&N,&F);
    int total;
    for (int i = 0;i < F;i++){
        int A;
        scanf("%d",&A);
        total += A;
    }

    //output
    if (total <= K*N){
        printf("%d\n",K*N-total);
    }else{
        printf("%d\n",-1);
    }
    return 0;
}