Prim's Algorithm incomplete

#define pii pair<int,int>
#define vpi vector<pii>
#define ff first
#define ss second

vpi V[3005];                             //graph is stored here as adjacency list
int vis[3005]={0};

int prim()
{
    priority_queue<pii> p;               //stored as <cost,node> pair
    int cost = 0;
    p.push(mp(0,0));                     //source is 0, and cost of source to source is 0
    while(!p.empty())
    {
        pii tem = p.top();
        p.pop();
        int v = tem.ss;
        if(!vis[v])
        {
            vis[v] = 1;
            cost += -tem.ff;
            For(i,V[v].sz)
                if(!vis[V[v][i].ff])
                    p.push(mp(-(V[v][i].ss),V[v][i].ff));
        }
    }
    return cost;
}