I want to display the number of negative numbers entered using functions

I'll help you on the way you're going, but there are several things you'd better do differently. One of the changes is that I would probably pass the argument by reference instead of making a return , but probably not yet learned to use it, so it goes in the simplest way.

#include <iostream>
#include <locale.h>

using namespace std;

int getnegativo(int a, int b);

int main() {
    setlocale(LC_ALL, "portuguese");
    int cont, cot = 0;

    do {
        cout << "digite um valor: ";
        cin >> cont;

      cot = getnegativo(cont, cot);

    } while (cont != -1);

    cout << "número de valores negativos digitados: " << cot;

    cout << endl << endl;
    return 0;
}

int getnegativo(int a, int b){
    if (a < 0)
        b++;
    return b;
}

See running on ideone .

I would start thinking about what you are using. Why use locale in every application, if it has no function in the code? It's still worse because it's something of C and not native to C ++.