const input = require('fs').readFileSync('/dev/stdin', 'utf8').split("\n");
if (input[0] === input[1]) { return console.log(0); }
const s = Array.from(input[0]);
const t = Array.from(input[1]);
let count = 0;
for (let i=0; i < s.length; i++) { 
    if (s[i] !== t[i]) { count++ };
}
console.log(count);
