#include <stdio.h>

int main()
{
    int k,n,f;
    int age[100];
    int i;
    int num;
    
    scanf("%d",&k);
    scanf("%d",&n);
    scanf("%d",&f);
    num=k*n;
    
    for(i=0;i<f;i++){
        scanf("%d",&age[i]);
        if((num-age[i]>0)){
            num-=age[i];
        }
        else{
            printf("-1\n");
            break;
        }
    }
    printf("%d\n",num);
    return 0;
}