"use strict";
    
const main = arg => {
    arg = arg.trim().split("\n");
    const S = arg[0].split("");
    const Srev = arg[0].split("").reverse();
    
    let diff = 0;
    
    for(let i in S) {
        if(S[i] !== Srev[i]) diff++;
    }
    
    // console.log(S);
    // console.log(Srev);
    
    console.log(diff / 2)
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));