#include <iostream>
using namespace std;
const long long MOD = 1000003;
 
long long mod(long long x) {
    long long r = x % MOD;
    return r < 0 ? r + MOD : r;
}
long long modsq(long long x) { return mod(x * x); }
long long pow(long long b, long long p) {
    if (p == 0) return 1;
    if (p == 1) return b;
    long long h = modsq(pow(b, p / 2));
    return (p & 1) ? mod(h * b) : h;
}
 
int main() {
    long long x;
    cin >> x;
    cout << pow(2, x) << endl;
}

If you guessed 253109, you're right. (Really? You probably just guessed "whatever the right answer after all that modding is". That's what I would do. It's okay.)

If you guessed 1.26765e+30, because

1. the compiler on the judge machine has different settings and/or versions from those of the compiler on your computer;
2. it silently links the math libraries into your namespace; and
3. it decides to choose its pow(double, double) over your pow(long long, long long) because (drum roll) 2 is an int, not a long long

you're also right.

Oops.

There you have it: a bug I couldn't find, or even reproduce or get warned about, on my own machine. (In my defense, I had about 5 minutes before the end of the contest in which this was an issue.)

I think the "correct" way to fix this in non-competition code would be to avoid

using namespace std;

but it's just too useful for competitions, so the only way is to stay away from names of built-ins...

From what I remember, I also got Compile Errors that didn't appear on my own computer for defining things called "end", "distance", and "hash". But those errors told me what the compiler complained about; the above was a Wrong Answer. Ouch.