why is it compulsory to use using namespace std in c++ language?

This is a simple program and my question is to know why is it compulsory to use using namespace std ? why program is not complied without using this ?

#include 

using namespace std;

int main(){

    int a , b , c , d;

    cout << "Enter First Value" <    cin >> a;

    cout << "Enter Second Value" <    cin >> b;

    cout << "Enter 1 to add values" << endl  << "Enter 2 to subtract values" <    cin >> c;

    if (c == 1){    
        d = a + b;
        cout << "After Adding your answer is " << d << endl;
    }

    else if (c == 2){
        d = a - b;
        cout << "After Subtracting your answer is " << d << endl;
    }

    else if (c == 3){
        d = a * b;
        cout << "After Multiplication your answer is " << d << endl;
    }
    else{
        cout << "You Enter Invalid operation to perform";   
    }

    system("pause");  //buitin function to stop the comand screen.

    return 0;
}