n = int(input())
count = 0
vector = list(input())

try:
    last = n - vector[::-1].index("R") - 1  # last red
    first = vector.index("W")  # first white

    while first < last:
        vector[first], vector[last] = vector[last], vector[first]
        count += 1
        last = n - vector[::-1].index("R") - 1
        first = vector.index("W")
    print(count)

except ValueError:
    print(0)