function main(input) {
	const args = input.split('\n')
    const str1 = args[0]
    const str2 = args[1]
    let cnt = 0
    for (let i=0, len = str1.length; i < len; i++) {
    	if (str1[i] !== str2[i]) cnt++
    }
  	console.log(cnt)
    
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));