class Solution {
public:
int pivotInteger(int n) {
int leftSum = 0;
int rightSum = 0;
int s = 1;
int e = n;
while(s <= e) {
if(leftSum + s < rightSum) leftSum += s++;
else rightSum += e--;
}
if(leftSum + s == rightSum) return s;
if(leftSum == rightSum + e) return e;
return -1;
}
};