'use strict';

function Main(input) {
    const tmp = input.trim().split('\n');
    const N = Number(tmp[0]);
    const S = tmp[1];
    // console.log(N, S);

    let result = 0;
    for (let i = 0; i < N; i++) {
        for (let j = 1; j < N; j++) {
            for (let k = 1; k < N; k++) {
                if (j === k) continue;
                if (i + j + k > N - 1) break;
                const check1 = [S[i], S[i + j], S[i + j + k]];
                // console.log(`check1${check1}`);
                if (check(check1)) {
                    result++;
                }
                const check2 = [S[i], S[i + k], S[i + j + k]];
                // console.log(`check2${check2}`)
                if (check(check2)) {
                    result++;
                }
            }
        }
    }
    console.log(result / 2);
}
function check(arr) {
    if (arr[0] !== arr[1] && arr[0] !== arr[2] && arr[1] !== arr[2]) {
        return true;
    }
    return false;
}

Main(require('fs').readFileSync('/dev/stdin', 'utf8'));

// Main(`4
// RRGB`);
// Main(`39
// RBRBGRBGGBBRRGBBRRRBGGBRBGBRBGBRBBBGBBB`);
