i have problem in finishing my code

Code:

`this program finds the largest multiple of 5 in a randomly generated Vector of 20 elements and returns it as well as its index. but What would biggest() return if none of the integers in the vector<> are multiple of five? and how can i correct it?`

#include <iostream>

#include <algorithm>

#include <vector>

#include <fstream>

using namespace std;

int biggest(vector<int>);

const int k=20;

int main()

{

ofstream fout("pasuxi.out");

vector <int> v(k);

for(int i=0; i<k; i++)

v[i]=rand() % 71+30;

fout<<"biggest is "<<v[biggest(v)]

<<". index= "<< biggest(v);

}

int biggest(vector<int> x)

{

int big=-1;

int index;

for(int i=0; i<x.size(); i++)

if(x[i]%5==0 && x[i]>big){

index=i;}

return index;

}