#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>

int main(void){
    long long int i,j=0,K,F,A[1000],N;
    scanf("%lld%lld%lld",&K,&N,&F);
    for(i=0;i<F;i++){scanf("%lld",&A[i]);}
    for(i=0;i<F;i++){j+=A[i];}
    if(K*N<j)printf("-1");
    if(K*N>=j)printf("%lld",K*N-j);
    
    return 0;
    }