/*
     AUTHOR : Ankit Kumar
     BRANCH : Information Technology
     INSTITUTE : Birla Institute of Technology
*/
#include<bits/stdc++.h>
using namespace std;
#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define pb push_back
#define MP make_pair
#define SET(a,val) memset(a,val,sizeof(a))
#define deci(n)         cout<<fixed<<setprecision(n);
#define fr(i,st,n)   for(int i=st;i<n;i++)
#define fr1(i,st,n) for(int i=st;i>=n;i--)
#define show(a,l,r) for(int i=l;i<r;i++) cout<<a[i]<<" ";
#define all(a)    a.begin(),a.end()
#define pi pair<int,int>
#define ff first
#define sc second
const int N=(int)100;
const int dx[] = {-1, 0, 1, 0, -1,-1, 1, 1};
const int dy[] = { 0, 1, 0,-1, -1, 1,-1, 1};
char dir[]={'L','D','R','U'};
//int dx[]={2,-2,1,-1,2,-2,1,-1};//for knights
//int dy[]={1,1,2,2,-1,-1,-2,-2};
typedef long long int ll;
typedef long double ld;
#define pll pair<ll,ll>
const ll mod=(ll)1e9+7;
const ll inf=1e18+100;
int BIT[100005],n;
ll prime[17],spf[N];
template<typename T>T lcm(T a, T b) {return a / gcd(a,b) * b;}
template<typename T> T min3(T a,T b,T c) {return min(a,min(b,c));}
template<typename T> T max3(T a,T b,T c) {return max(a,max(b,c));}
bool isVowel(char ch){ ch=toupper(ch); if(ch=='A'||ch=='U'||ch=='I'||ch=='O'||ch=='E') return true; return false;}
ll mInv(ll a, ll m)
{
    ll m0 = m;
    ll y = 0, x = 1;
    if (m == 1)
      return 0;
    while (a > 1)
    {
        ll q = a / m;
        ll t = m;
        m = a % m, a = t;
        t = y;
        y = x - q * y;
        x = t;
    }
    if (x < 0)
       x += m0;
return x;
}
ll power(ll x, ll y)
{
    ll res = 1;
    x = x % mod;
    while (y > 0)
    {
        if (y & 1)
            res = (res*x) % mod;
        y = y>>1;
        x = (x*x)% mod;
    }
    return res;
}
ll gcd(ll a,ll b)
{
    if(b==0)
        return a;
    return gcd(b,a%b);
}
long long Extendedgcd (long long a, long long b, long long *x, long long *y) {
    if (!a) {
        *x = 0;
        *y = 1;
        return b;
    }
    long long nx, ny;
    long long d = Extendedgcd (b % a, a, &nx, &ny);
    long long k = b / a;
    *y = nx;
    *x = -k * nx + ny;
    return d;
}
void update(int x, int delta)
{
    if(!x)
    {
        BIT[x]+=delta;
        return;
    }
      for(; x <= N; x += x&-x)
        BIT[x] += delta;
}
ll query(int x)
{
     ll sum = BIT[0];
     for(; x > 0; x -= x&-x)
        sum += BIT[x];
     return sum;
}
void Sieve()
{
    for(int i=1;i<=N;i++)
        prime[i]=1;
    prime[0]=0;
    prime[1]=0;
    for (ll p=2; p*p<=N; p++)
    {
        if (prime[p] == 1)
        {
            for (ll i=p*p; i<=N; i += p)
                prime[i] = 0;
        }
    }
    for(int p=3;p<=N;p++)
        prime[p]+=prime[p-1];
}
void sieve()
{
    spf[1] = -1;
    for (int i=2; i<N; i++)
        spf[i] = i;
    for (int i=4; i<N; i+=2)
        spf[i] = 2;
    for (ll i=3; i*i<N; i++)
    {
        if (spf[i] == i)
        {
            for (ll j=i*i; j<N; j+=i)
                if (spf[j]==j)
                    spf[j] = i;
        }
    }
}
struct cmp {
    bool operator() (const pair<int,int>&x, const pair<int,int>&y) const {
        if(x.first!=y.first)
        return x.first<y.first;
        return x.second>y.second;
    }
};
void gInput(int m,vector<int>adj[])
{
    int u,v;
    for(int i=0;i<m;i++)
    {
        cin>>u>>v;
        adj[u].pb(v);
        adj[v].pb(u);
    }
}
vector<ll>fac;
void factorial()
{
    int N=(int)1e5+100;
    fac.resize(N);
    fac[0]=1;
    for(ll i=1;i<N;i++)
    {
        fac[i]=((fac[i-1]%mod)*i)%mod;
    }
}
ll nCr(ll n,ll r)
{
    ll x=fac[n];
    ll y=fac[r];
    ll z=fac[n-r];
    ll pr=((y%mod)*(z%mod))%mod;
    pr=mInv(pr,mod);
    ll ncr=((x%mod)*(pr%mod))%mod;
    return ncr;
}
                         /******************Lets Code*******************/

int main()
{
    fast
    int t=1;
    //cin>>t;
    while(t--)
    {
        int n;
        cin>>n;
        string s;
        cin>>s;
        int w[n]={},r[n]={};
        w[n-1]=s[n-1]=='W';
        r[n-1]=s[n-1]=='R';
        fr1(i,n-2,0)
        {
            if(s[i]=='W')
            {
                w[i]=w[i+1]+1;
                r[i]=r[i+1];
            }
            else
            {
                r[i]=r[i+1]+1;
                w[i]=w[i+1];
            }
        }
        int c=0;int ans=INT_MAX;
        fr(i,0,n)
        {
            int res=0;
            res+=min(c,r[i]);
            int p=c-res;
            res+=min(r[i]-res,w[i]+res);
            res+=p;
            ans=min(ans,res);
            c+=(s[i]=='W');
            //cout<<ans<<" "<<p<<"\n";
        }
        cout<<ans<<"\n";
    }
    return 0;
}

Measured memory_bytes: 5348