Help me in solving PPSCPP78 problem

help
,
1

My issue

how to use mod in this question.

My code

#include <bits/stdc++.h>
using namespace std;

int main() {
	// your code goes here 
	int a,b,sum,pro,result;
	cin>>a>>b;
	sum=a+b;
	pro=a*b;
	result=pro-sum;
	if(result>=0)
	cout<<result;
	else
	cout<< -result;

}

Learning course: Introduction to C++
Problem Link: Coding problem - 1 Practice Problem in Introduction to C++ - CodeChef