梯形面積

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include<stdbool.h>

int main()
{
	int upn, downn, hn;
	int area;
	scanf("%d%d%d", &upn, &downn, &hn);
	area = (upn + downn) * hn / 2;
	printf("%d", area);
}