Area Calculator

This is a simple ish code to calculate the area of the chosen shape I'll add any formula/shape that people ask me to but for now I'll add the basic ones.

This is in C++ btw also if anyone know how I can syntax highlight these that'd be great.

#include <iostream>
#include <cmath>
int main(){
    int input;
    std::cout << "Curently available:\n"<< "1. Triangle\n" << "2. Triangle2\n" << "3. Rectangle\n"<< "4. Square\n" << "5. Regular Pentagon\n" << "Please input the desired shape:\n";
    std::cin >> input;
    switch(input) {
    case 1:
        float area, p, a, b, c;
        std::cout << "Please input the lenghts of the triangle: ";
        std::cin >> a >> b >> c;
        p = (a+b+c)/2;
        if(0>a || 0>b || 0>c || 0>p*(p-a)*(p-b)*(p-c)){
            std::cout << "This is an invalid triangle.\n";
        } else {
            area = sqrt(p*(p-a)*(p-b)*(p-c));
            std::cout << area << "\n";
        }
    break;
        case 2:
        float area2, a2, b2;
        std::cout << "Please input the height and base of the triangle: ";
        std::cin >> a2 >> b2;
        area2 = a2*b2/2;
        if(0>a2 || 0>b2){
            std::cout << "This is an invalid triangle.\n";
        }else {
            std::cout << area2 << "\n";
        }
        break;
            case 3:
            float area1, a1, b1;
            std::cout << "Please input the lenghts of the rectangle: ";
            std::cin>>a1>>b1;
            area1 = a1*b1;
            if(0>a1 || 0>b1){
            std::cout << "This is an invalid rectangle.\n";
            } else{
            std::cout <<area1<<"\n";
            }
            break;
                case 4:
                float area4, a4;
                std::cout << "Please input the lenght of the side: ";
                std::cin >> a4;
                area4 = a4*a4;
                if(a4>0){
                    std::cout << area4 << "\n";
                } else {
                    std::cout << "This is an invalid square.\n";
                }
                break;
                    case 5:
                    float area3, a3;
                    std::cout << "Please input the lenght of the side: ";
                    std::cin >> a3;
                    area3 = 1.72*a3*a3;
                    if(a3>0){
                        std::cout << area3 << "\n";
                    } else{
                        std::cout << "This is an invalid regular pentagram, please re-check it. \n";
                    }
        default:
        break;
    }
    return 0;
}