B'William has a favorite bracket sequence. Since his favorite sequence is quite big he provided it to you as a sequence of positive integers c_1, c_2, ... , c_n where c_i is the number of consecutive brackets "(" if i is an odd number or the number of consecutive brackets ")" if i is an even number. For example for a bracket sequence "((())()))" a corresponding sequence of numbers is [3, 2, 1, 3] . You need to find the total number of continuous subsequences (subsegments) [l, r] ( l <= r ) of the original bracket sequence, which are regular bracket sequences. A bracket sequence is called regular if it is possible to obtain correct arithmetic expression by inserting characters "+" and "1" into this sequence. For example, sequences "(())()", "()" and "(()(()))" are regular, while ")(", "(()" and "(()))(" are not. The first line contains a single integer n (1 <= n <= 1000) , the size of the compressed sequence. The second line contains a sequence of integers c_1, c_2, ... , c_n (1 <= c_i <= 10^9) , the compressed sequence. Output a single integer -- the total number of subsegments of the original bracket sequence, which are regular bracket sequences. It can be proved that the answer fits in the signed 64-bit integer data type. In the first example a sequence (((()(()))( is described. This bracket sequence contains 5 subsegments which form regular bracket sequences: In the second example a sequence ()))(()(()))) is described. In the third example a sequence ()()(()) is described. '...