#include <iostream>
#include <vector>

int main(){
  int n;
  std::cin >> n;
  std::vector<int> d_in(n-1); for( int& v : d_in ){ v=0; }
  std::vector<int> d_out(n-1); for( int& v : d_out ){ v=0; }
  int in = 0, out = 0, min = 0;
  for( int i=0; i<n-1; ++i ){
    std::cin >> d_in[i] >> d_out[i];
    in += d_in[i];
    out += d_out[i];
  }
  //min = std::max( 0, in - out );
  for( int i=0; i<n-1; ++i ){
    min += std::max( 0, d_in[i] - out + d_out[i] );
  }
  std::cout << std::max( in - std::max( min, std::max( 0, in - out ) ) + 1, 0 ) << std::endl; 
}