Seg Fault on Cin? [closed]

  int n;cin>>n;
  vector<string> bfs;
  vector<int> visited;
  vector<vector<int>> adj(n,vector<int>(n,0));
  int range=n-1;
  for (int i=0;i<range;i++){
    cout<<"hi"<<"\n";
    int a,b;cin>>a>>b;
    cout<<"hi";
    adj[a-1][b-1]=1;
    cout<<"hi";
    adj[b-1][a-1]=1;
    cout<<"hi";
  }

This is my code, for some reason I get a segfault on the very last input?