const fs = require("fs");
// const { mainModule } = require("process");
const filePath = process.platform === "linux" ? "/dev/stdin" : "./input.txt";
let input = fs.readFileSync(filePath).toString().split("\n");
solution(+input[0]);
function solution(N) {
let 한수=0;
for (let i = 1; i <= N; i++) {
if (i < 100) {
한수++;
}
let A = Number(String(i)[1]) - Number(String(i)[0]);
let B = Number(String(i)[2]) - Number(String(i)[1]);
if (A === B) {
한수++;
}
}
console.log(한수);
}