var obj = init();
Main();
function nextInt(){return myconv(next(),1);}
function nextStrArray(){return myconv(next(),2);}//半角スペース分割
function nextIntArray(){return myconv(next(),4);}//半角スペース + 数値化
function nextCharArray(){return myconv(next(),6);}//1文字分割
function next(){return obj.next();}
function hasNext(){return obj.hasNext();}
function myin(){return require("fs").readFileSync("/dev/stdin", "utf8").trim().split("\n");}
function init(){
  var inputFile = myin();
  var returnObj = {
    "list" : inputFile, "index" : 0, "max" : inputFile.length,
    "hasNext" : function(){return (this.index < this.max);},
    "next" : function(){if(!this.hasNext()){throw "ArrayIndexOutOfBoundsException これ以上ないよ";}else{var returnInput = this.list[this.index];this.index++;return returnInput;}}
  };
  return returnObj;
}
function myout(t){console.log(t);}//標準出力
function myerr(t){console.error(t);}//標準エラー出力
//[no]要素の扱い。数値型
//不明値、異常時:引数そのまま返す　1:数値へ変換
//2:半角SPで分割 4:半角SPで分割し、数値配列へ
//6:1文字で分割  7:1文字で分割し、数値配列へ
//8:半角SPで結合 9:改行で結合 0:文字なしで結合
function myconv(i,no){try{switch(no){case 1:return parseInt(i);case 2:return i.split(" ");case 4:return i.split(" ").map((a)=>Number(a));case 6:return i.split("");case 7:return i.split("").map((a)=>Number(a));case 8:return i.join(" ");case 9:return i.join("\n");case 0:return i.join("");default:return i;}}catch(e){return i;}}
function Main(){
  var N = nextInt();
  var s = next();
  var rlist = new Array(N).fill(0);
  var glist = new Array(N).fill(0);
  var blist = new Array(N).fill(0);
  for(var i = N - 1; i > 0; i--){
    if(s[i] == "R"){
      rlist[i]++;
    }
    if(s[i] == "G"){
      glist[i]++;
    }
    if(s[i] == "B"){
      blist[i]++;
    }
    rlist[i - 1] += rlist[i];
    glist[i - 1] += glist[i];
    blist[i - 1] += blist[i];
  }
  //myerr(rlist);
  var output = 0;
  for(var i = 0; i < N - 2; i++){
    for(var j = i + 1; j < N - 1; j++){
      if(s[i] != s[j]){
        //i,jより右にある,s[i]でもs[j]でもないものをカウントする
        if(s[i] == "R" && s[j] == "B" || s[i] == "B" && s[j] == "R"){
          output += glist[j + 1];
          if(s[j + (j - i)] == "G"){
            output--;
          }
        }
        if(s[i] == "R" && s[j] == "G" || s[i] == "G" && s[j] == "R"){
          output += blist[j + 1];
          if(s[j + (j - i)] == "B"){
            output--;
          }
        }
        if(s[i] == "G" && s[j] == "B" || s[i] == "B" && s[j] == "G"){
          output += rlist[j + 1];
          if(s[j + (j - i)] == "R"){
            output--;
          }
        }
      }
    }
  }
  myout(output);
}
