#include<stdio.h>
main(){
    int K,N,F;
    scanf("%d%d%d",&K,&N,&F);
    int sum = K*N;
    for(int i = 0;i < F;i++){
        int n;
        scanf("%d",&n);
        sum -= n;
    }
    if(sum < 0){
        printf("-1");
    }else{
        printf("%d",sum);
    }
}