Bus

passengers = int(input())
stations = int(input())

for i in range(1, stations + 1):
    output_passengers = int(input())
    input_passengers = int(input())

    passengers += input_passengers - output_passengers

    if i % 2 == 1:
        passengers += 2
    else:
        passengers -= 2

print(f"The final number of passengers is : {passengers}")