Where is the simple error?

Hello, i'm going mad because I can't find the problem in this simple code. Why do this don't work?

Code:

float promedi(a,b)
{
return ((a+b)/2);
}

main()
{
float x,y;
printf("Dame dos valores\n");
scanf("%f%f", &x, &y);
printf("%f",promedi(x,y));
system("pause");
}

I got at least two different errors (strange long number solutions, window must close...).

Thanks in advance.

 

When it says 'Dame dos valores' it means tell me two numbers.