package yukicoder;

import java.util.Scanner;

public class Yuki10 {
	
	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);
		int k = scan.nextInt();
		int n = scan.nextInt();
		int f = scan.nextInt();
	    int[] age = new int[f];
	    
	    for(int a = 0; a < age.length; a++ ) {
	    	age[a] = scan.nextInt();
	    }
	    scan.close();
	    
	    int beanTotal = k * n;
	    Boolean success  = true;
	    for(int a = 0; a < age.length; a++ ) {
	    	
	    	if(beanTotal<age[a]){
	    		success = false;
	    		break;
	    	}
	    	beanTotal = beanTotal-age[a]; 
	    }
	    
	    if (success == true) {
	    	System.out.println(beanTotal);
	    } else {
	    	System.out.println(-1);	    	
	    }
	}	
}