// dfmt off
T lread(T=long)(){return readln.chomp.to!T;}T[] lreads(T=long)(long n){return iota(n).map!((_)=>lread!T).array;}
T[] aryread(T=long)(){return readln.split.to!(T[]);}void arywrite(T)(T a){a.map!text.join(' ').writeln;}
void scan(L...)(ref L A){auto l=readln.split;foreach(i,T;L){A[i]=l[i].to!T;}}alias sread=()=>readln.chomp();
void dprint(L...)(lazy L A){debug{auto l=new string[](L.length);static foreach(i,a;A)l[i]=a.text;arywrite(l);}}
alias PQueue(T,alias l="b<a")=BinaryHeap!(Array!T,l);import std, core.bitop;
// dfmt on
void main()
{
    long N = lread();
    auto S = sread();
    auto R = new long[](N + 1);
    auto W = new long[](N + 1);
    foreach (i; 0 .. N)
    {
        R[i + 1] = R[i] + (S[i] == 'R');
        W[i + 1] = W[i] + (S[i] == 'W');
    }
    long ans = long.max;
    foreach (i; 0 .. N + 1)
    {
        long r = R[$ - 1] - R[i];
        long w = W[i];
        long a = min(r, w) + abs(r - w);
        ans = ans.min(a);
    }
    writeln(ans);
}
