#include <stdio.h>

int main(void)
{
	int n, a[99], b, c, sum = 0, ans = 0, i;
	scanf("%d", &n);
	for (i = 0; i < n - 1; ++i) scanf("%d", &a[i]);
	for (i = 0; i < n - 1; ++i) {
		scanf("%d%d", &b, &c);
		sum += c - b;
		ans += sum * a[i];
	}
	printf("%d", ans);
	return 0;
}