int lire_entier() {
    int n = 0;
    char c;
    printf("entier> ");
    while ( (c = getchar()) != '\n' && c != EOF) {
        if ('0' <= c && c <= '9') {
            n = n*10 + (c-'0');
        } else {
          while ( (c = getchar()) != '\n' && c != EOF);
          return -1;
        }
    }
    return n;
}