อ่านและตรวจสอบ input

#include <stdio.h>

int main(void) {
    int age;
    printf("Age: ");
    if (scanf("%d", &age) != 1 || age < 0) {
        puts("Invalid age");
        return 1;
    }
    printf("Next year: %d\n", age + 1);
    return 0;
}

#include <iostream>

int main() {
    int age;
    std::cout << "Age: ";
    if (!(std::cin >> age) || age < 0) {
        std::cerr << "Invalid age\n";
        return 1;
    }
    std::cout << "Next year: " << age + 1 << '\n';
}