public class Solution {

public int FourSumCount(int[] A, int[] B, int[] C, int[] D) {

var map = new Dictionary<int, int>();

foreach (var a in A) {

foreach (var b in B) {

var temp = a + b;

if (map.ContainsKey(temp)) {

++map[temp];

} else {

map[temp] = 1;

}

}

}

var res = 0;

foreach (var c in C) {

foreach (var d in D) {

var target = -1 * (c + d);

if (map.ContainsKey(target)) {

res += map[target];

}

}

}

return res;

}

}