public class Ex04 {
	public static void main(String[] args){

		int[] num = new int[3];

		for(int j=0; j<args.length; j++){
			num[j] = Integer.parseInt(args[j]);
		}
		
		int k = num[0];
		int n = num[1];
		int f = num[2];
		
		int sum = 0;
		int beans = 0;
		int result = 0;
		int[] a = new int[f];

		
		
		int[] nums = new int[f];

		for(int i=0; i<f; i++){
			nums[i] = Integer.parseInt(args[i]);
			sum += nums[i];
		}
		
		
		
		
		/*for(int i=4; i<f; i++){
			a[i-4] = Integer.parseInt(args[i]);
			sum += a[i-4];
		}*/

		beans = k * n;
		result = beans - sum;

		if(result<0){
			System.out.println("-1");
		}else{
			System.out.println(result);
		}

	}
}