#include <stdio.h>

int main(){
	int c, result;
	int value = 0;
	int vxod[999];
	int a = 0;
	for(int i = 0; i < 4; i++){
		c = scanf("%d",&value);;
		vxod[i] = c;
		a = i;
	}
	value = 0;
	do{
		result = vxod[0] * vxod[value] + 1;
		value++;	
	}while(value < a);
	printf("%d\n", result);
	return 0;
}