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#include<iostream>
using namespace std;
 
 
int main(){
    int n;
    int k;
    cin >> n >> k;
    int flag=0;
    vector<int> a(n);
    for(int a_i = 0;a_i < n;a_i++){
       cin >> a[a_i];
    }
    for (int i=0; i<n;i++)
        {
        for(int j=i+1; j<n;j++)
        {
 
        if((a[i]+a[j])%k==0)
        flag++;
    }
   }    
    cout << flag;
    return 0;
}

prog.cpp:10:5: error: use of undeclared identifier 'vector'
    vector<int> a(n);
    ^
prog.cpp:10:15: error: expected '(' for function-style cast or type construction
    vector<int> a(n);
           ~~~^
prog.cpp:10:17: error: use of undeclared identifier 'a'
    vector<int> a(n);
                ^
prog.cpp:12:15: error: use of undeclared identifier 'a'
       cin >> a[a_i];
              ^
prog.cpp:19:13: error: use of undeclared identifier 'a'
        if((a[i]+a[j])%k==0)
            ^
prog.cpp:19:18: error: use of undeclared identifier 'a'
        if((a[i]+a[j])%k==0)
                 ^
6 errors generated.

Standard output is empty