Simple addition/substraction calculator

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#define ended(X) (X == '\n' || X == '\r' || X == '\0')
#define SIZE 100
int main (void)
{
    double ans = 0, tmp;
    char line[SIZE];
    fgets(line, SIZE, stdin);
    char *endptr, *currptr = line;
    do
    {
        ans += tmp = strtod(currptr, &endptr);
        if (!tmp && currptr == endptr)
            return fputs("Invalid input", stderr);
        if (errno == ERANGE)
            return fputs("Out of range", stderr);
        currptr = endptr;
    }
    while (!ended(*endptr) && endptr - line + SIZE > 0);
    printf("= %g", ans);
    return 0;
}