'use strict'
function main(input) {
    const S = input.split("");
    const dict = new Map();
    for (let char of S) {
        if (!dict.has(char)) {
            dict.set(char, 1);
        } else {
            dict.set(char, dict.get(char) + 1);
        }
    }
    const dictArr = Array.from(dict);
    const Nkey = dictArr.map((d) => d[0]).length;
    const counts = dictArr.map((d) => d[1]);
    if (counts.filter((count) => count === 2).length === Nkey) {
        console.log('Yes');
    } else {
        console.log('No');
    }
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));