extern int __VERIFIER_nondet_int();
void abort() { };
                                                                                                                                                          
                                                                                 

int fibo1(int n);
int fibo2(int n);

/*@ 
    requires (1);
    ensures (((\result == 0) || (0 < n)));
@*/
int fibo1(int n) {
    if (n < 1) {
        return 0;
    } else if (n == 1) {
        return 1;
    } else {
        return fibo2(n-1) + fibo2(n-2);
    }
}

/*@ 
    requires (1);
    ensures ((((((((((2 == \result) && (3 == n)) || ((\result == 1) && (n == 1))) || (5 < n)) || ((3 == \result) && (n == 4))) || (n < 1)) || ((5 == \result) && (5 == n))) || ((\result == 1) && (2 == n))) && (((\result == 0) || (n == 1)) || (1 < n))));
@*/
int fibo2(int n) {
    if (n < 1) {
        return 0;
    } else if (n == 1) {
        return 1;
    } else {
        return fibo1(n-1) + fibo1(n-2);
    }
}

// fibo 1-30
// 1, 1, 2, 3, 5,
// 8, 13, 21, 34, 55, 
// 89, 144, 233, 377, 610,
// 987, 1597, 2584, 4181, 6765,
// 10946, 17711, 28657, 46368, 75025,
// 121393, 196418, 317811, 514229, 832040

int main() {
    int x = 6;
    int result = fibo1(x);
    if (result != 8) {
        ERROR: {/*@ assert(0); */;abort();}
    }
    return 0;
}