def calculate_time(a, b, n, times):
    total_time=0
    cycle_time=a+b
    for t in times:
        arrival_time=t%cycle_time
        if arrival_time>=a:
            wait_time=cycle_time-arrival_time
            total_time+=wait_time
    return total_time
a, b=map(int, input().split())
n=int(input())
times=list(map(int, input().split()))
result=calculate_time(a, b, n, time)
print(result)