#include <iostream>
#include "math.h"
#include <limits>
#include <string>
using namespace std;
double f(double x);
double countIntegral(double a, double b, int n);
template <typename T>
void input(T &a, string b);
void checkAandB(double a, double b);
int main() {
    double y, a, b;
    int n;
    input(a, "a");
    input(b, "b");
    input(n, "n");
    checkAandB(a, b);
    y = countIntegral(a, b, n);
    cout << y << endl;
    system("pause");
    return 0;
}
double f(double x) {
    return pow(sin(x), 2) - 3 * cos(x);
}
double countIntegral(double a, double b, int n) {
    double h, s = 0, x;
    int i;
    h = (b - a) / n;
    for (i = 0; i < n - 1; i++) {
        s += f(a + h*(i + 0.5));
    }
    s *= h;
    return s;
}
template <typename T>
void input(T &a, string b) {
    while (true) {
        cout << "Enter " << b << endl;
        cin >> a;
        if (cin.get() == '\n' && !cin.fail()) {
            break;
        }
        else if (cin.fail()) {
            cout << "Number is too big/small" << endl;
        }
        else {
            cout << "Wrong input" << endl;
        }
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }
}
void checkAandB(double a, double b) {
    while (a >= b) {
        cout << "A should be less than B" << endl;
        input(a, "a");
        input(b, "b");
    }
}