#include<stdio.h>
int main(void){
int K, N, F;
int A[100];
int i, goukei = 0;
scanf ("%d%d%d", &K, &N, &F);

for ( i = 0; i < F; i++){
	scanf ("%d", &A[i]);
	goukei += A[i];
}

if ( K * N >= goukei)
	printf ( "%d", K * N - goukei);
else
	printf ("-1");

	return 0;
}