class Solution {
public:
int Fibonacci(int n) {

    if(n>39) return -1;
    int f=0,g=1;
    if (n==0) return 0;
    if (n==1) return 1;
    n=n-1;
    while(n)
    {
        g=f+g;
        f=g-f;
        n--;
    }
    return g;

}

};