#include<stdio.h>

int main(void){
    int K,N,F,i;
    int wa=0;
    int A[100];

    scanf("&d %d %d",&K,&N,&F);
    for(i=1;i<=F;++i){
        scanf("%d",&A[i]);
        wa=wa+A[i];
    }    
    if(K*N-wa>=0){
        printf("%d",K*N-wa);
    }else{
        printf("-1");
    }
}