60分求调！

#include <iostream>
using namespace std;
int main(){
    int n;
    unsigned long long a = 1, b = 2, c;
    cin >> n;
    if (n == 1){
        c = 1;
    }
    else if (n == 2){
        c = 2;
    }
    for (int i = 3; i <= n; i ++){
        c = a + b;
        a = b; b = c;
    }
    cout << c;
}