练习：求斐波那契数列的第n项。f(1) = 1, f(2) = 1, f(3) = 2, f(n) = f(n-1) + f(n-2)。
int main()
{
int n;
cin >> n;

int a = 1, b = 1, i = 1;
while (i < n)
{
    int c = a + b;
    a = b;
    b = c;
    i ++ ;
}

cout << a << endl;

return 0;

}