方法一:
T(n) = 2 ^ n - 1

#include <iostream>
using namespace std;

int main() {
    int n;
    long long step = 1;
    cin >> n;

    int k = 1;
    while (k <= n) {
        step *= 2;
        k ++;
    }
    cout << step - 1 << endl;
    return 0;
}

方法二:

#include <iostream>
using namespace std;

int main() {
    int n;
    long long step = 1;
    cin >> n;

    if (n == 0) {
        cout << "0" << endl;
        return 0;
    }

    int k = 2;
    while (k <= n) {
        step = step * 2 + 1;
        k ++;
    }
    cout << step << endl;
    return 0; 
}