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#include <iostream>
using namespace std;
 
int main ()
{
	unsigned long long int n, sum=0, m;
	cin >> n;
    m= abs (n);
	sum= (1 + m) * m / 2;
	if (n>0)
		cout << sum;
	if (n==0)
		cout << 1;
	else
	{
		sum= *-1;
		cout << sum + 1;
	}
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:8:14: error: call of overloaded ‘abs(long long unsigned int&)’ is ambiguous
     m= abs (n);
              ^
In file included from /usr/include/c++/6/cstdlib:75:0,
                 from /usr/include/c++/6/ext/string_conversions.h:41,
                 from /usr/include/c++/6/bits/basic_string.h:5417,
                 from /usr/include/c++/6/string:52,
                 from /usr/include/c++/6/bits/locale_classes.h:40,
                 from /usr/include/c++/6/bits/ios_base.h:41,
                 from /usr/include/c++/6/ios:42,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from prog.cpp:1:
/usr/include/stdlib.h:735:12: note: candidate: int abs(int)
 extern int abs (int __x) __THROW __attribute__ ((__const__)) __wur;
            ^~~
In file included from /usr/include/c++/6/ext/string_conversions.h:41:0,
                 from /usr/include/c++/6/bits/basic_string.h:5417,
                 from /usr/include/c++/6/string:52,
                 from /usr/include/c++/6/bits/locale_classes.h:40,
                 from /usr/include/c++/6/bits/ios_base.h:41,
                 from /usr/include/c++/6/ios:42,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/6/cstdlib:185:3: note: candidate: __int128 std::abs(__int128)
   abs(__GLIBCXX_TYPE_INT_N_0 __x) { return __x >= 0 ? __x : -__x; }
   ^~~
/usr/include/c++/6/cstdlib:180:3: note: candidate: long long int std::abs(long long int)
   abs(long long __x) { return __builtin_llabs (__x); }
   ^~~
/usr/include/c++/6/cstdlib:172:3: note: candidate: long int std::abs(long int)
   abs(long __i) { return __builtin_labs(__i); }
   ^~~
prog.cpp:16:10: error: invalid type argument of unary ‘*’ (have ‘int’)
   sum= *-1;
          ^

Standard output is empty