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

#define NDEBUG
#include <cassert>


typedef long long ll;
typedef long double Double;
typedef unsigned long long ull;
typedef pair<int,int> ii;
typedef pair<ll,ll> llll;
typedef pair<double,double> dd;

typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<ii> vii;
typedef vector<vector<ii>> vvii;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<llll> vllll;
typedef vector<bool> vb;
typedef vector<string> vs;
typedef vector<double> vd;
typedef vector<long double> vD;

#define sz(a)  int((a).size())
#define pb  push_back
#define eb  emplace_back
#define FOR(var,from,to) for(int var=(from);var<=(to);++var)
#define rep(var,n)  for(int var=0;var<(n);++var)
#define rep1(var,n)  for(int var=1;var<=(n);++var)
#define repC2(vari,varj,n)  for(int vari=0;vari<(n)-1;++vari)for(int varj=vari+1;varj<(n);++varj)
#define repC3(vari,varj,vark,n)  for(int vari=0;vari<(n)-2;++vari)for(int varj=vari+1;varj<(n)-1;++varj)for(int vark=varj+1;vark<(n);++vark)
#define ALL(c)  (c).begin(),(c).end()
#define RALL(c)  (c).rbegin(),(c).rend()
#define tr(i,c)  for(auto i=(c).begin(); i!=(c).end(); ++i)
#define found(s,e)  ((s).find(e)!=(s).end())
#define mset(arr,val)  memset(arr,val,sizeof(arr))
#define mid(x,y) ((x)+((y)-(x))/2)
#define IN(x,a,b) ((a)<=(x)&&(x)<=(b))
#define cons make_pair
#define clamp(v,lo,hi) min(max(v,lo),hi)
#define ABS(x) max((x),-(x))

template<typename T1, typename T2> inline void amin(T1 & a, T2 const & b) { if (a>b) a=b; }
template<typename T1, typename T2> inline void amax(T1 & a, T2 const & b) { if (a<b) a=b; }
template<typename X, typename T> auto vectors(X x, T a) { return vector<T>(x, a); }
template<typename X, typename Y, typename Z, typename... Zs> auto vectors(X x, Y y, Z z, Zs... zs) { auto cont = vectors(y, z, zs...); return vector<decltype(cont)>(x, cont); }

inline ll square(ll x) { return x * x; }
inline ll gcd(ll a, ll b) { while(a) swap(a, b%=a); return b; }
inline ll lcm(ll a, ll b) { return a/gcd(a,b)*b; }
template <typename T>
inline T mod(T a, T b) { return ((a % b) + b) % b; }

template <typename T>
int find_left(vector<T>& v, T elem) {
    return (int)(upper_bound(v.begin(), v.end(), elem) - v.begin()) - 1;
}
template <typename T>
int find_right(vector<T>& v, T elem) {
    return (int)(lower_bound(v.begin(), v.end(), elem) - v.begin());
}

const ll MOD=1000000007LL;

inline ll ADD(ll x, ll y) { return (x+y) % MOD; }
inline ll SUB(ll x, ll y) { return (x-y+MOD) % MOD; }
inline ll MUL(ll x, ll y) { return x*y % MOD; }
inline ll POW(ll x, ll e) { ll v=1; for(; e; x=MUL(x,x), e>>=1) if (e&1) v = MUL(v,x); return v; }
inline ll INV(ll y) { /*assert(y%MOD!=0);*/ return POW(y, MOD-2); }
inline ll DIV(ll x, ll y) { return MUL(x, INV(y)); }

#define INTSPACE 12
char _buf[INTSPACE*1000000 + 3];

int loadint() {
    if (fgets(_buf, INTSPACE+3, stdin)==NULL) return 0;
    return atoi(_buf);
}

int loadvec(vector<int>& v, int N=-1) {
    if (N == 0) {
        v.clear();
        return 0;
    }
    if (N == -1) {
        N = loadint();
        if (N==0) return 0;
    }
    int bufsize = INTSPACE*N + 3;
    if (fgets(_buf, bufsize, stdin)==NULL) return 0;
    v.resize(N);

    int i=0;
    bool last = false;
    for (char *p=&_buf[0]; ;) {
        char *q = p;
        while (*q > ' ') ++q;
        if (*q == 0x0D || *q == 0x0A) last = true;
        *q = 0;
        v[i++] = atoi(p);
        if (last || i == N) break;
        p = q+1;
    }
    return i;
}
void read_cr() {
    fgets(_buf, 256, stdin);
}

void horizontal(vector<int>& v) {
    int L = v.size();
    for (int i=0; i<L; ++i) {
        printf("%d%c", v[i], (i==L-1)?'\n':' ');
    }
}
void horizontall(vector<long long>& v) {
    int L = v.size();
    for (int i=0; i<L; ++i) {
        printf("%lld%c", v[i], (i==L-1)?'\n':' ');
    }
}

ll solve(int N, string& s) {
    vi r(N,0), g(N,0), b(N,0);
    switch (s[0]){
        case 'R': r[0]++; break;
        case 'G': g[0]++; break;
        case 'B': b[0]++; break;
    }
    for(int i=1;i<N;++i){
        switch (s[i]){
            case 'R':
                r[i] = r[i-1]+1;
                g[i] = g[i-1];
                b[i] = b[i-1];
                break;
            case 'G':
                r[i] = r[i-1];
                g[i] = g[i-1]+1;
                b[i] = b[i-1];
                break;
            case 'B':
                r[i] = r[i-1];
                g[i] = g[i-1];
                b[i] = b[i-1]+1;
                break;
        }
    }
    vll rg(N,0),rb(N,0),gr(N,0),gb(N,0),br(N,0),bg(N,0);
    for(int i=1;i<N;++i){
        switch (s[i]){
            case 'R':
                gr[i] = gr[i-1] + g[i-1];
                br[i] = br[i-1] + b[i-1];
                rg[i] = rg[i-1];
                bg[i] = bg[i-1];
                rb[i] = rb[i-1];
                gb[i] = gb[i-1];
                break;
            case 'G':
                gr[i] = gr[i-1];
                br[i] = br[i-1];
                rg[i] = rg[i-1] + r[i-1];
                bg[i] = bg[i-1] + b[i-1];
                rb[i] = rb[i-1];
                gb[i] = gb[i-1];
                break;
            case 'B':
                gr[i] = gr[i-1];
                br[i] = br[i-1];
                rg[i] = rg[i-1];
                bg[i] = bg[i-1];
                rb[i] = rb[i-1] + r[i-1];
                gb[i] = gb[i-1] + g[i-1];
                break;
        }
    }
    vll rgb(N,0),rbg(N,0),grb(N,0),gbr(N,0),brg(N,0),bgr(N,0);
    for(int i=2;i<N;++i){
        switch (s[i]){
            case 'R':
                gbr[i] = gbr[i-1] + gb[i-1];
                bgr[i] = bgr[i-1] + bg[i-1];
                rbg[i] = rbg[i-1];
                brg[i] = brg[i-1];
                rgb[i] = rgb[i-1];
                grb[i] = grb[i-1];
                break;
            case 'G':
                gbr[i] = gbr[i-1];
                bgr[i] = bgr[i-1];
                rbg[i] = rbg[i-1] + rb[i-1];
                brg[i] = brg[i-1] + br[i-1];
                rgb[i] = rgb[i-1];
                grb[i] = grb[i-1];
                break;
            case 'B':
                gbr[i] = gbr[i-1];
                bgr[i] = bgr[i-1];
                rbg[i] = rbg[i-1];
                brg[i] = brg[i-1];
                rgb[i] = rgb[i-1] + rg[i-1];
                grb[i] = grb[i-1] + gr[i-1];
                break;
        }
    }
    ll ans = gbr[N-1] + bgr[N-1] + rbg[N-1] + brg[N-1] + rgb[N-1] + grb[N-1];


    for (int w=1; w*2<=N-1; ++w) {
        for(int i=0; i+w*2<=N-1; ++i){
            int j=i+w, k=j+w;
            if (s[i]==s[j] || s[i]==s[k] || s[j]==s[k]) continue;
            --ans;
        }
    }

    return ans;
}

int main() {
    int N;cin>>N;
    string s;cin>>s;
    cout << solve(N,s) << endl;
    return 0;
}


Measured memory_bytes: 3784