C++ :: Output Numbers From A Set Of List Not Used

This is one part of my program, I need a loop that checks through the input(specifically the agent numbers) and then outputs the agent numbers that aren't in the input, from 1 to 20(because there should only be 20 agent).

My input is :
1 3 250.00
2 0 0
15 1 1000.00
3 4 300.00
12 2 500.00
1 2 300.00
3 4 115.00
21 3 400.00
-1 4 250.00
15 1 200.00
9 5 -150.00
18 2 140.00
13 2 550.00

the first numbers are the agent numbers so 1 , 2 ,15 are agent numbers and out output for agents who didnt participate are : 2 , 5 , 6 , 7, 8, 9, 10 , 11 , 14 , 16 , 17 ,19 20

this is the code ive wrote but its not working.

void not_part() {
ins.open(in_file);
int i=0;
int sum=0;
cout<<"AGENTS WHO DID NOT PARTICIPATE IN THE CAMPAIGN"<<endl;
cout<<fixed<<showpoint;

[code].....