C lang

#include <stdio.h>

int isLeapYear(int year) {
    return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
}

int countLeapYears(int year) {
    return (year / 4) - (year / 100) + (year / 400);
}

int main() {
    int X, Y;
    scanf("%d %d", &X, &Y);
    int leapYears = countLeapYears(Y) - countLeapYears(X - 1);
    int totalYears = Y - X + 1;
    int totalDays = totalYears * 365 + leapYears;
    
    printf("%d\n", totalDays);
    return 0;
}