#include <stdio.h>

int main(void){
    int K,N,F,i,A,T=0,T1=0,T0=0;
    
    scanf("%d%d%d",&K,&N,&F);
    for(i = 0; i < F; i++){
        scanf("%d",&A);
        T += A;
    }
    T1 = K*N;
    T0 = T1-T;
    if(T < T1){
        printf("%d",T0);
    }else{
        printf("-1");
    }
}