# 设每头牛每天吃草量为 1
def calculate_new_growth():
    total_grass_1 = 15 * 20
    total_grass_2 = 20 * 10
    days_diff = 20 - 10
    growth_per_day = (total_grass_1 - total_grass_2) // days_diff
    return growth_per_day

print(calculate_new_growth())