Can someone help with the code below?

#include

#include

using namespace std;

int validateUserInput(int firstNum, int secondNum);

void oddNumbers(int firstNum, int secondNum);

int sumEvenNumbers(int firstNum, int secondNum);

int sumSqureOddNumbers(int firstNum, int secondNum);

void displayUppercaseLetters();

int main()

{

int firstNum, secondNum, sumEven, sumSquareOdd;

char ch;

while (true)

{

while (true)

{

cout << " Enter two positive integer numbers." << endl;

cout << "First number must be less than the second number:" << endl;

cout << "Enter numbers:";

cin >> firstNum;

cin >> secondNum;

if (cin.fail())

{

cout << "Incorrect Input." << endl;

cout << "Please try again." << endl;

cin.clear();

cin.ignore(256, ' ');

continue;

}

else if(validateUserInput(firstNum, secondNum) == -1)

{

cout<<"No negative numbers!"<

cout<<"Please try again."<

}

else if (validateUserInput(firstNum, secondNum) == 0)

{

cout << "First number must be less than the second number!" << endl;

cout << "Please try again." << endl;

continue;

}

else

break;

}

oddNumbers(firstNum, secondNum);

sumEven = sumEvenNumbers(firstNum, secondNum);

cout << "Sum of even integers between " << firstNum << " and " << secondNum << " = "

<< sumEven << endl;

sumSquareOdd = sumSqureOddNumbers(firstNum, secondNum);

cout << "Sum of the squares of odd integers between " << firstNum << " and " << secondNum

<< " = " << sumSquareOdd << endl;

displayUppercaseLetters();

cout << " Do you want to repeat this program?" << endl;

cout << "y/n" << endl;

cin >> ch;

if (ch == 'y' || ch == 'Y')

{

continue;

}

else

{

break;

}

}

return 0;

}

int validateUserInput(int firstNum, int secondNum)

{

if(firstNum<0 || secondNum<0)

return -1;

else if (firstNum < secondNum)

return 1;

else

return 0;

}

void oddNumbers(int firstNum, int secondNum)

{

cout << "Odd integers between " << firstNum << " and " << secondNum << " are:" << endl;

int i = firstNum;

while (i <= secondNum)

{

if (i % 2 != 0)

{

cout << i << " ";

}

i++;

}

cout << endl;

}

int sumEvenNumbers(int firstNum, int secondNum)

{

int i = firstNum, tot = 0;

while (i <= secondNum)

{

if(i%2==0)

{

tot += i;

}

i++;

}

return tot;

}

int sumSqureOddNumbers(int firstNum, int secondNum)

{

int tot = 0;

int i = firstNum;

cout << "Number Square of Number" << endl;

while (i <= secondNum)

{

if (i % 2 != 0)

{

tot += i * i;

}

cout << i << "\t" << i * i << endl;

i++;

}

return tot;

}

void displayUppercaseLetters()

{

cout << "Upper case letters are: ";

int i = 65;

while (i <= 90)

{

cout << (char)i << " ";

i++;

}

}

I keep getting an error

||error: ld returned 1 exit status|