#include <iostream>
using namespace std;
int main(){
    int n;  cin >> n;
    if(n < 0)   n = -n;
    cout << n << endl;
    return 0;
}

直接abs

#include <iostream>
#include <cmath>
using namespace std;
int main(){
    int n;  cin >> n;
    cout << abs(n) << endl;
    return 0;
}