#include<stdio.h>
int main()
{
    int k,n,f,i,ans,sum=0;
    scanf("%d %d %d",&k,&n,&f);
    int a[f];
    for(i=0;i<f;i++){
        scanf("%*c%d",&a[i]);
        sum+=a[i];
    }
    ans=(k*n)-sum;
    if((k*n)<sum){
        printf("-1\n");
    }else{
        printf("%d\n",ans);
    }
    return 0;
}