#include <bits/stdc++.h>
using namespace std;

int dp[55][2];

int main() {
	ios::sync_with_stdio(false); cin.tie(0);
	string str; cin >> str; int len = str.length();
	dp[0][0] = str[0] == 'R' ? 0 : 1;
	dp[0][1] = str[0] == 'R' ? 1 : 0;
	for (int i = 1; i < len; ++i) {
		dp[i][0] = dp[i-1][0] + (str[i] == 'R' ? 0 : 1);
		dp[i][1] = min(dp[i-1][1], dp[i-1][0]) + (str[i] == 'R' ? 1 : 0);
	}
	cout << min(dp[len-1][0], dp[len-1][1]) << endl;
	return 0;
}

ac代码...有很多情况的....