C++ quiz. In case of the following type conversions, what is

In case of the following type conversions, what is the output of the program?

#include <iostream>

int main() {
    float a = 256.5;
    unsigned char b = static_cast<unsigned char>(a);
    a += b + 1.f;
    
    std::cout << static_cast<int>(a);
}

257.5
257
256.5
256
The result is undefined
Compilation error

Explanation

Floating-integral conversions
An rvalue of a floating-point type can be converted to an rvalue of an integer type. The conversion truncates; that is, the fractional part is discarded. The behavior is undefined if the truncated value cannot be represented in the destination type.