K, N, F = map(int, input().split())
age_list = list(map(int, input().split()))

def bean(K, N, F, age_list):
    if K*N-sum(age_list) >= 0:
        return K*N-sum(age_list)
    else:
        return -1
        
print(bean(K, N, F, age_list))