Calculator in C - if-statement not working anymore after one calculation

I am trying to write a calculator program in C which only allows digits. If a calculation is finished, the program should ask if you want to calculate again.

If you type a character for an operand the calculator should print that the operand is not valid.

That works so far, but if I type a character after the first calculation my if-statement (to print that the operand is not valid) is not working anymore.

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

int main() {
    float operand1, operand2, result;
    char operator;
    char YN; // yes/no

    do {
      printf("Enter a formula (+, -, *, / are possible):\n");
      scanf("%f %c %f",&operand1, &operator, &operand2);
      if (operand1!=(int)operand1 || operand2!=(int)operand2) {
        printf("operand not valid!\n");
        return 0;
      }
      scanf("%c", &YN);
        
      switch (operator) { 
        case '+':
          result = operand1 + operand2;
          printf("The sum of %.0f and %.0f is %f\n", operand1, operand2, result);
          break;
        case '-':
          result = operand1 - operand2;
          printf("The difference of %.0f and %.0f is %f\n", operand1, operand2, result);
          break;
        case '*':
          result = operand1 * operand2;
          printf("The product of %.0f and %.0f is %f\n", operand1, operand2, result);
          break;
        case '/':
          if (operand2 != 0) {
            result = operand1 / operand2;
            printf("The quotient of %.0f and %.0f is %f\n", operand1, operand2, result);
          } else {
            printf("operand2 cant be 0.\n");
          }
          break;
        case '%':
          printf("The remainder of .0%d and %.0d is %d\n", (int)operand1, (int)operand2, (int)result);
          break;
        case 'M':
          if (operand1 > operand2) {
            result = operand1;
            printf("The maximum of %.0f and %.0f is %f\n", operand1, operand2, result);
          } else if (operand1 < operand2) {
            result = operand2;
            printf("The maximum of %.0f and %.0f is %f\n", operand1, operand2, result);
          } else {
            printf("The values are the same.\n");
          }
          break;
        case 'm':
          if (operand1 < operand2) {
            result = operand1;
            printf("The minimum of %.0f and %.0f is %f\n", operand1, operand2, result);
          } else if (operand1 > operand2) {
            result = operand2;
            printf("The minimum of %.0f and %.0f is %f\n", operand1, operand2, result);
          } else {
            printf("The values are the same.\n");
          }
          break;
        default:
          printf("Operator '%c' not valid!\n", operator);
          return 0;
      }

    printf("Do you want to calculate something else? Y/N):\n");
    scanf("%c", &YN);
    fflush(stdin);
    } while (YN == 'y' || YN == 'Y');
    
    return 0;
}

Problem:

The if-statment to check if the operands are integers is not working anymore after the first calculation.

        if (operand1!=(int)operand1 || operand2!=(int)operand2) {
            printf("operand not valid!\n");
            return 0;
        }

e.g. Input:

2+3

it prints - The sum of 2 and 3 is 5

Do you want to calculate something else?

y

b+1

prints - The sum of 1 and 3 is 4

But it should print - operand not valid!

It works if you type a character in the first calculation.

I really need your help. Could you please fix my code so that the if-statement is also working for multiple calculations. Your help is very much appreciated.