#include<stdio.h>
#include<math.h>
int main()
{
    double a, b, area;
    scanf("%lf %lf", &a, &b);
    area = a * b / 2;
    if (floor(area) == area) {
        printf("s=%d", (int)area);
    } else {
        printf("s=%g", area);
    }
    return 0;
}

本题需强制转换整数类型进行计算

#include <iostream>
using namespace std;

int main()
{
    int d, h;
    scanf("%d %d", &d, &h);

    cout << "s=" << ((float)d * (float)h) / 2 << endl;

    return 0;
}