#include<stdio.h>

int main(void){
    int K,N,F,i,a,wa;

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